Wiki app improvements including performance fixes to document revision view.

Radon complexity changes:

    radon cc -a -e "apps/wiki/migrations/*,apps/wiki/tests/*" apps/wiki/

Before:
428 blocks (classes, functions, methods) analyzed.
Average complexity: A (3.81308411215)

After:
443 blocks (classes, functions, methods) analyzed.
Average complexity: A (3.7381489842)

The document revision view locally with all revisions (e.g. https://developer-local.allizom.org/en-US/docs/Web/HTML$history?limit=all)

Before: 664 SQL queries
After: 22 SQL queries

The same view with the default pagination (https://developer-local.allizom.org/en-US/docs/Web/HTML$history)

Before: 66 SQL queries
After: 19 SQL queries


Indeed, we'll have to look out if this isn't a performance drag on restarts, but I doubt it.