Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/channels/db/migrations/postgres/000049_create_channels.down.sql

@@ -0,0 +1,61 @@
+CREATE INDEX IF NOT EXISTS idx_channels_name ON channels (name);
+
+ALTER TABLE channels DROP COLUMN IF EXISTS shared;
+
+DROP INDEX IF EXISTS idx_channels_scheme_id;
+
+DO $$
+<<modify_column_type_if_type_is_different>>
+DECLARE
+    col_exist_and_type_different boolean := false;
+BEGIN
+    SELECT count(*) != 0 INTO col_exist_and_type_different
+    FROM information_schema.columns
+    WHERE table_name = 'channels'
+    AND table_schema = current_schema()
+    AND column_name = 'groupconstrained'
+    AND NOT data_type = 'boolean';
+
+    IF col_exist_and_type_different THEN
+        ALTER TABLE channels ALTER COLUMN groupconstrained TYPE boolean;
+    END IF;
+END modify_column_type_if_type_is_different $$;
+
+ALTER TABLE channels DROP COLUMN IF EXISTS groupconstrained;
+
+CREATE INDEX IF NOT EXISTS idx_channels_txt ON channels (name,displayname);
+
+ALTER TABLE channels DROP COLUMN IF EXISTS schemeid;
+
+CREATE INDEX IF NOT EXISTS idx_channels_displayname ON channels (displayname);
+
+DO $$
+<<modify_column_type_if_type_is_different>>
+DECLARE
+    col_exist_and_type_different boolean := false;
+BEGIN
+    SELECT count(*) != 0 INTO col_exist_and_type_different
+    FROM information_schema.columns
+    WHERE table_name = 'channels'
+    AND table_schema = current_schema()
+    AND column_name = 'purpose'
+    AND data_type = 'character varying'
+    AND NOT character_maximum_length = 64;
+
+    IF col_exist_and_type_different THEN
+        ALTER TABLE channels ALTER COLUMN purpose TYPE varchar(64);
+    END IF;
+END modify_column_type_if_type_is_different $$;
+
+DROP INDEX IF EXISTS idx_channels_displayname;
+DROP INDEX IF EXISTS idx_channels_txt;
+DROP INDEX IF EXISTS idx_channels_displayname_lower;
+DROP INDEX IF EXISTS idx_channels_name_lower;
+DROP INDEX IF EXISTS idx_channels_name;
+DROP INDEX IF EXISTS idx_channels_update_at;
+DROP INDEX IF EXISTS idx_channels_team_id;
+DROP INDEX IF EXISTS idx_channels_delete_at;
+DROP INDEX IF EXISTS idx_channels_create_at;
+DROP INDEX IF EXISTS idx_channel_search_txt;
+
+DROP TABLE IF EXISTS channels;