We remove the remaining special casing for **model.User
and add unit tests to lock in the behavior.
Additional load tests were done locally to confirm
there are no hidden code paths left out.
https://mattermost.atlassian.net/browse/MM-59947
```release-note
NONE
```
UserStore.GetAllProfiles is a very frequent call. We cache it
when there are no options passed which is the default case.
https://mattermost.atlassian.net/browse/MM-57084
```release-note
NONE
```
It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.
https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.
For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```
