Add "SSO login link" on the user login page

Thanks. This looks like a "why wasn't this done before?" type of thing.

(Likely just because noone explicitly requested it until now / I didn't bother to think of a 'generalized' solution. But

• if it helps the simplesamlphp_auth people coming on board, that's enough reason.
• Also, a link with a CSS class actually looks like a general enough solution.)

I'm going to use this issue to muck around with the Gitlab comment and merge system -- which I haven't used much yet. I'm seeing something that likely should be changed, but I'll likely do that myself and merge it. (And, in a break with this module's history, I might even add a test.)

Personal conclusion: there are some fairly basic things which I was yeats behind on:

• Using gitlab to review and merge MRs.
• Translatability of configuration values. This module doesn't do that properly yet! (It should be translating the existing menu item titles too.) I'll fix that separately.
• Purging something like a form from the cache, when configuration is re-saved. (I'm sure I have dug into it before, but now I'll remember.)

Thank you for this addition!