Add DB enums (#20601)

* Add DB enums

```release-note
Upgrade note:
Several schema changes are made which imposes additional
DB constraints to make the data more strict.

Here are the times taken:

Postgres (131869 channels, 2 teams):
[bigdb] # CREATE TYPE channel_type AS ENUM ('P', 'G', 'O', 'D');
CREATE TYPE
Time: 14.114 ms
[bigdb] # ALTER TABLE channels alter column type type channel_type using type::channel_type;
ALTER TABLE
Time: 3856.790 ms (00:03.857)
[bigdb] # CREATE TYPE team_type AS ENUM ('I', 'O');
CREATE TYPE
Time: 4.191 ms
[bigdb] # ALTER TABLE teams alter column type type team_type using type::team_type;
ALTER TABLE
Time: 116.205 ms
[bigdb] # CREATE TYPE upload_session_type AS ENUM ('attachment', 'import');
CREATE TYPE
Time: 4.266 ms
[bigdb] # ALTER TABLE uploadsessions alter column type type upload_session_type using type::upload_session_type;
ALTER TABLE
Time: 37.099 ms

MySQL (270959 channels, 2 teams):
mysql> ALTER TABLE Channels MODIFY COLUMN Type ENUM("D", "O", "G", "P");
Query OK, 270959 rows affected (13.24 sec)
Records: 270959  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE Teams MODIFY COLUMN Type ENUM("I", "O");
Query OK, 2 rows affected (0.04 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE UploadSessions MODIFY COLUMN Type ENUM("attachment", "import");
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

All the commands were tested on a 8 core, 16GB RAM machine.
```

https://mattermost.atlassian.net/browse/MM-45508

```release-note
NONE
```

* Added migration list

```release-note
NONE
```

db/migrations/postgres/000090_create_enums.down.sql

@@ -0,0 +1,44 @@
+ALTER TABLE channels alter column type type varchar(1);
+
+DO
+$$
+BEGIN
+  IF EXISTS (SELECT * FROM pg_type typ
+                            INNER JOIN pg_namespace nsp ON nsp.oid = typ.typnamespace
+                        WHERE nsp.nspname = current_schema()
+                            AND typ.typname = 'channel_type') THEN
+    DROP TYPE channel_type;
+  END IF;
+END;
+$$
+LANGUAGE plpgsql;
+
+ALTER TABLE teams alter column type type varchar(255);
+
+DO
+$$
+BEGIN
+  IF EXISTS (SELECT * FROM pg_type typ
+                            INNER JOIN pg_namespace nsp ON nsp.oid = typ.typnamespace
+                        WHERE nsp.nspname = current_schema()
+                            AND typ.typname = 'team_type') THEN
+    DROP TYPE team_type;
+  END IF;
+END;
+$$
+LANGUAGE plpgsql;
+
+ALTER TABLE uploadsessions alter column type type varchar(32);
+
+DO
+$$
+BEGIN
+  IF EXISTS (SELECT * FROM pg_type typ
+                            INNER JOIN pg_namespace nsp ON nsp.oid = typ.typnamespace
+                        WHERE nsp.nspname = current_schema()
+                            AND typ.typname = 'upload_session_type') THEN
+    DROP TYPE upload_session_type;
+  END IF;
+END;
+$$
+LANGUAGE plpgsql;

db/migrations/postgres/000090_create_enums.up.sql

@@ -0,0 +1,44 @@
+DO
+$$
+BEGIN
+  IF NOT EXISTS (SELECT * FROM pg_type typ
+                            INNER JOIN pg_namespace nsp ON nsp.oid = typ.typnamespace
+                        WHERE nsp.nspname = current_schema()
+                            AND typ.typname = 'channel_type') THEN
+    CREATE TYPE channel_type AS ENUM ('P', 'G', 'O', 'D');
+  END IF;
+END;
+$$
+LANGUAGE plpgsql;
+
+ALTER TABLE channels alter column type type channel_type using type::channel_type;
+
+DO
+$$
+BEGIN
+  IF NOT EXISTS (SELECT * FROM pg_type typ
+                            INNER JOIN pg_namespace nsp ON nsp.oid = typ.typnamespace
+                        WHERE nsp.nspname = current_schema()
+                            AND typ.typname = 'team_type') THEN
+    CREATE TYPE team_type AS ENUM ('I', 'O');
+  END IF;
+END;
+$$
+LANGUAGE plpgsql;
+
+ALTER TABLE teams alter column type type team_type using type::team_type;
+
+DO
+$$
+BEGIN
+  IF NOT EXISTS (SELECT * FROM pg_type typ
+                            INNER JOIN pg_namespace nsp ON nsp.oid = typ.typnamespace
+                        WHERE nsp.nspname = current_schema()
+                            AND typ.typname = 'upload_session_type') THEN
+    CREATE TYPE upload_session_type AS ENUM ('attachment', 'import');
+  END IF;
+END;
+$$
+LANGUAGE plpgsql;
+
+ALTER TABLE uploadsessions alter column type type upload_session_type using type::upload_session_type;