* materialize PublicChannels table
Introduce triggers for each supported database that automatically maintain a subset of the Channels table corresponding to only public channels. This improves corresponding queries that no longer need to filter out 99% DM channels.
This initial commit modifies the channel store directly for easier code reviewing, but the next wraps an experimental version around it to enable a kill switch in case there are unforeseen performance regressions.
This addresses [MM-11886](https://mattermost.atlassian.net/browse/MM-11886) and [MM-11945](https://mattermost.atlassian.net/browse/MM-11945).
* extract the experimental public channels materialization
Wrap the original channel store with an experimental version that
leverages the materialized public channels, but can be disabled to
fallback to the original implementation.
This addresses MM-11947.
* s/ExperimentalPublicChannelsMaterialization/EnablePublicChannelsMaterialization/
* simplify error handling
* move experimental config listener until after store is initialized
* 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.
* 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.
