7011 - Render logic for signup link on login page

[poikilotherm]

What this PR does / why we need it:

Users should not get confused about a non-existing, error 401 prone signup link on the login page.

Which issue(s) this PR closes:

Closes #7011

Special notes for your reviewer:
The underlying function has been used in various other places (e. g. the header UI, hidding the very same link there), so this is really straight forward.

Suggestions on how to test this:

1. Deploy
2. Disable signup
3. Goto Login page and check if link is not rendered anymore.
4. Enable signup
5. Goto Login page and check if link is rendered again.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:
Yes. Please see the opening of #6170, where the initial link has been mocked. This is simply hidden again when signup is disabled.

Is there a release notes update needed for this change?:
I don't think so. It's a small fix.

Additional documentation:
Nada.

[coveralls]

Coverage remained the same at 19.557% when pulling 6575273 on poikilotherm:7011-render-signup into b97185a on IQSS:develop.

[mheppler]

👍 Looks good. The render logic used for sign up link in the navbar is being applied here.