Provide nicer access to siteconfig settings in templates.

Templates accessed siteconfig settings through accessing
`siteconfig.settings.<name>`. However, this didn't handle default
values for the siteconfig.

There isn't a great way of implementing that for templates, so instead,
we have a wrapper that handles defaults, accessible through the
`siteconfig_settings` context variable. All templates should be updated
to use this instead.

The settings field is also no longer read-only in the admin UI, since
it's now safe these days to modify a JSONField within the admin UI form.

Reviewed at https://reviews.reviewboard.org/r/5693/