Now that we allow document review UIs in the diff viewer, we've been

having to deal with the possibility of extension static media being

loaded multiple times from different sources onto a page. Currently,

both extensions and review UIs can load static media onto a page. It's

possible that an extension loads static media onto the diff viewer page, and

that the extension has a review UI that loads the same static media onto

the page, resulting in duplicate loads. At best this is redundant and

costly, and at worst this can break things if the static media sets

state that needs to be shared among the extension and any review UIs.

The latter is the case for Power Pack's document review.
This change adds a ReviewUI.load_static_media_inline class variable

that lets review UIs indicate whether to load static media when the

review UI is rendered inline. If the extension managing the review UI

already loads its static media in the diff viewer page or any other page

where a Review UI might be rendered, then this can be set to False

to prevent duplicate loading. This change also improves some of the

review UI documentation. When I land this change I'll take care to merge

the changes with the review UI doc improvements in /r/14542/.
We did explore more general solutions to handle duplicate static media loads

at the extension or page level, which would be useful if we ever have more

things that can load static media, but couldn't work out a good solution.

Since extensions and review UIs are the only things in Review Board that

deal with loading static media, this fix is sufficient for now.

Built and checked the docs.
Used with a change in Power Pack that solves problems from

  duplicate static media loads.