Extracted logout to the login service so the logic can be shared instead
of re-implemented at each stage. For this, the SocialAuthService was
split so the driver management is in its own class, so it can be used
elsewhere without use (or circular dependencies) of the
SocialAuthService.
During review of #4467
- Moved preference views to more general "my-account" area.
- Started new layout for my-account with sidebar.
- Added MFA to prefeences view (to be moved).
- Moved "common" template partials, that are only used in layouts, to
layouts/parts folder.
- Simplified HTML structure of header template.
- Extracted logo and links from header template to simplify.
- Added header-links-start template for easier extension/customization
without needing to override full list of links.
- Added test to cover usage of this.
For #4564
