Don't error out if a datagrid is sorted on a missing column.

At some point we had mistakenly marked the "people" column as sortable, which
doesn't really make sense. We've since fixed this, but if users had "people"
left in their sort preferences, it would cause an uncaught exception. This adds
a none-test and logs the failure.

Testing done: Could load /r/ using my old db.

Reviewed at https://reviews.reviewboard.org/r/5688/
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/