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/000043_thread_memberships.up.sql

@@ -0,0 +1,14 @@
+CREATE TABLE IF NOT EXISTS threadmemberships(
+    postid VARCHAR(26) NOT NULL,
+    userid VARCHAR(26) NOT NULL,
+    following boolean,
+    lastviewed bigint,
+    lastupdated bigint,
+    PRIMARY KEY (postid, userid)
+);
+
+ALTER TABLE threadmemberships ADD COLUMN IF NOT EXISTS unreadmentions bigint;
+
+CREATE INDEX IF NOT EXISTS idx_thread_memberships_last_update_at ON threadmemberships(lastupdated);
+CREATE INDEX IF NOT EXISTS idx_thread_memberships_last_view_at ON threadmemberships(lastviewed);
+CREATE INDEX IF NOT EXISTS idx_thread_memberships_user_id ON threadmemberships(userid);