MM-8796: Full implementation of "Schemes" in Store/Model/App layers. (#8357)

* Add Scheme model and stub store.

* Port ChannelStore to be Scheme aware.

* Make almost all the API/APP layer work with ChannelSchemes.

Only thing still hacky is UpdateChannelMemberRoles().

* Add basic SchemeStore implementation.

* Migrate UpdateChannelMemberRoles properly and fix tests.

* Update store tests and mocks so they work.

* Include creating default roles in Scheme create store function.

* Implement role deletion and start scheme deletion.

* Only use non-deleted roles for authorization.

* Add GetByScheme method to Team store.

* Add GetChannelsByScheme.

* Update store mocks.

* Implement scheme deletion in the store.

* Rename is valid function.

* Add offset and limit to queries to fetch teams and channels by scheme.

* Fix queries.

* Implement scheme awareness in Team store and add a migration.

* Tidy up ChannelStore mapping functions and add exhaustive unit tests.

* Add all missing i18n.

* Proper tests for TeamStore internal functions and fix them.

* Make additional TeamMember fields nullable.

* Make new ChannelMember fields nullable.

* Create new nullable columns without defaults.

* Make new fields in large tables nullalble.

* Fix empty list of TeamMembers.

* Deduplicate SQL queries.

* Fix spelling.

* Fix review comment.

* More review fixes.

* More review fixes.

app/channel_test.go

@@ -120,7 +120,7 @@ func TestJoinDefaultChannelsCreatesChannelMemberHistoryRecordTownSquare(t *testi
 
 	// create a new user that joins the default channels
 	user := th.CreateUser()
-	th.App.JoinDefaultChannels(th.BasicTeam.Id, user, model.CHANNEL_USER_ROLE_ID, "")
+	th.App.JoinDefaultChannels(th.BasicTeam.Id, user, false, "")
 
 	// there should be a ChannelMemberHistory record for the user
 	histories := store.Must(th.App.Srv.Store.ChannelMemberHistory().GetUsersInChannelDuring(model.GetMillis()-100, model.GetMillis()+100, townSquareChannelId)).([]*model.ChannelMemberHistoryResult)
@@ -146,7 +146,7 @@ func TestJoinDefaultChannelsCreatesChannelMemberHistoryRecordOffTopic(t *testing
 
 	// create a new user that joins the default channels
 	user := th.CreateUser()
-	th.App.JoinDefaultChannels(th.BasicTeam.Id, user, model.CHANNEL_USER_ROLE_ID, "")
+	th.App.JoinDefaultChannels(th.BasicTeam.Id, user, false, "")
 
 	// there should be a ChannelMemberHistory record for the user
 	histories := store.Must(th.App.Srv.Store.ChannelMemberHistory().GetUsersInChannelDuring(model.GetMillis()-100, model.GetMillis()+100, offTopicChannelId)).([]*model.ChannelMemberHistoryResult)