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/scheme.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2018-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import "github.com/mattermost/mattermost-server/model"
+
+func (a *App) GetScheme(id string) (*model.Scheme, *model.AppError) {
+	if result := <-a.Srv.Store.Scheme().Get(id); result.Err != nil {
+		return nil, result.Err
+	} else {
+		return result.Data.(*model.Scheme), nil
+	}
+}