move remaining "theme" settings from settings.py dictionary to SiteMode model

[tobiasmcnulty]

Lilia would like the ability to edit various strings on the site without re-deploying and without engaging a developer. This PR moves the remaining settings from SITE_THEMES into the SiteMode model.

[tobiasmcnulty]

@ejucovy Could you give this a review when you have a moment?

[ejucovy]

Is there any way to stash this on the class instead of the instance? Or even on the get_site_mode function object? If I'm following correctly, even after this we'll still be making N queries (instead of ~5N) when we're rendering a template with N questions on it.

[tobiasmcnulty]

Good catch! I would like to avoid caching anything on a Python object that may persist in the interpreter outside the request/response cycle (as doing so may lead to some difficult to troubleshoot invalidation issues), however, we already have the site mode in the template context so it's just a matter of figuring out how to get it into the Submission model methods (or how to refactor them/put them elsewhere so that's not required).

For the time being, I added a template tag which pre-fills _cached_site_mode with the SITE_MODE available in the template context, along with a test to make sure the link doesn't get broken.

Longer term, this probably needs some refactoring, but I think this will work for now.

[tobiasmcnulty]

@ejucovy Do you want to take another look at this? I think I've addressed all your feedback. I may go ahead and merge in an hour or two if I haven't heard back so Lilia can test / get production content ready. We can take up any issues you find as new work if that's okay (please do make sure I implemented your suggested changes appropriately!).

[ejucovy]

All looks good to me!