Speed up the New Review Request page and cache Tool lookup for fewer queries.

The New Review Request page can be very slow with large numbers of
repositories. We were requesting far more information than needed, as we
only showed the name, so now we request only the name from the database
when displaying the repository in the list.

The big slowdown, however, was due to the Tool lookups. We grabbed the
SCMTool from every repository, which required loading the Tool entry for it
from the database. If there were 100 Git repositories, this meant we'd have 100
fetches of the Tool from the database. Clearly too inefficient and unnecessary.

Now we have a small caching layer preventing redundant lookups. Tool
is associated with a ToolManager that provides a special QuerySet that
sticks all items fetched through get() into a database. We precompute this
cache by doing an initial all() if the cache is empty, further reducing
lookups.

This cache is never cleared, except by restarting the server process.
However, Tool objects never (or should never) change, except when registered
during an rb-site upgrade, at which point the server should be restarted
anyway. We can add intelligent clearing of the cache if need be later, but
it shouldn't be an issue.

Ran profiling on the New Review Request page. On my deployment of 20
repositories, it went from a total of 27 database queries on the page
to 8. The only Tool query it did was the initial cache population,
meaning we saved 19 queries (number of repositories - 1).

Unit tests passed.