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.

api4/team_test.go

@@ -1672,7 +1672,7 @@ func TestUpdateTeamMemberRoles(t *testing.T) {
 
 	// user 1 (team admin) tries to demote system admin (not member of a team)
 	_, resp = Client.UpdateTeamMemberRoles(th.BasicTeam.Id, th.SystemAdminUser.Id, TEAM_MEMBER)
-	CheckBadRequestStatus(t, resp)
+	CheckNotFoundStatus(t, resp)
 
 	// user 1 (team admin) demotes system admin (member of a team)
 	th.LinkUserToTeam(th.SystemAdminUser, th.BasicTeam)
@@ -1698,7 +1698,7 @@ func TestUpdateTeamMemberRoles(t *testing.T) {
 
 	// user 1 (team admin) tries to promote a random user
 	_, resp = Client.UpdateTeamMemberRoles(th.BasicTeam.Id, model.NewId(), TEAM_ADMIN)
-	CheckBadRequestStatus(t, resp)
+	CheckNotFoundStatus(t, resp)
 
 	// user 1 (team admin) tries to promote invalid team permission
 	_, resp = Client.UpdateTeamMemberRoles(th.BasicTeam.Id, th.BasicUser.Id, "junk")