admin: Add log out button

[BBaoVanC]

Fixes #552

Adds a logout button to the header, in the top right.

Tasks:

• [ ] CHANGES.rst

[BBaoVanC]

Wow, the admin CSS is a horrible mess right now. is it better to just do as little change here (like I already have) so it works, and then make another pull request to clean up the CSS?

[ix5]

Wow, the admin CSS is a horrible mess right now. is it better to just do as little change here (like I already have) so it works, and then make another pull request to clean up the CSS?

Re: Admin CSS: Yes, please open another PR for that and keep this PR focused.

Re: Clearing the cookies: I'm not quite sure whether we need to match SameSite and Secure attributes from server when clearing cookies. Also, IMO we can use a bit more "modern" JS and modern Interfaces such as CookieStore in the admin (just keep the client JS as backward-compatible as possible).

[ix5]

@BBaoVanC would you mind applying the PR feedback? I know it's a bit much to ask, but we should at least try to keep track of SameSite and Secure stuff, I think.

[jelmer]

Please rebase this PR after the force-push.

[BBaoVanC]

Please rebase

Done

[BBaoVanC]

Is there anything else this PR needs? I believe when I last looked at it I had already applied @ix5's changes.