feat(providers): add Mattermost provider (#6290)
* added mattermost provider
* upaded docs and removed callback uri
* username -> name
* updated types to match other providers
* cleanup
* add logos
* style sign-in button
* add JSDoc to profile
* add JSDoc comments
* correction
Co-authored-by: Balázs Orbán <info@balazsorban.com>
delete mattermost icons
