Add a `signin` or `login` option for location of notices

Created by: jjlin

Feature request description

Currently, notices can have a location of either top or home. I'd like to be able to add a notice only on the sign-in page.

Is your feature request related to a problem? If so, please describe.

I currently have allowSignup disabled. When someone visits Sourcegraph, the sign-in page says "To create an account, contact the site admin." This is not terribly helpful since the visitor may have no idea who the site admin is.

Describe alternatives you've considered.

One alternative would be to allow setting contact info for the site admin and displaying that as part of the message on the sign-in page. This would solve my particular problem, but it's not as general of a solution as adding a signin or login location for notices. Of course, these options aren't mutually exclusive either.

I'm currently working around this issue by injecting my notice via htmlBodyTop in the critical config, but this displays the notice on every single page, which isn't ideal.