* 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.
* AllowEditPost and PostEditTimeLimit migration
* Not set EDIT_POST permission to sysadmin_role if ALLOW_EDIT_POST is configured to NEVER
* Remove a bit of code duplication
* XYZ-13: Update Permission and Role structs to new design.
* XYZ-10: Role store.
* XYZ-9/XYZ-44: Roles API endpoints and WebSocket message.
* XYZ-8: Switch server permissions checks to store backed roles.
* XYZ-58: Proper validation of roles where required.
* XYZ-11/XYZ-55: Migration to store backed roles from policy config.
* XYZ-37: Update unit tests to work with database roles.
* XYZ-56: Remove the "guest" role.
* Changes to SetDefaultRolesFromConfig.
* Short-circuit the store if nothing has changed.
* Address first round of review comments.
* Address second round of review comments.
