Re-add a 'fetch logs' link to the new views-driven repo admin UI

When we tossed the old, crappy admin UI and replaced it with shiny views, we lost the ability to trigger a manual log fetch from the repo admin UI. This reintroduces that behavior in the API itself. It includes:

• New VCAPI-level menu item callbacks that trigger log fetching and lock clearing
• An updated default view that includes the new field
• Another little one-off views field handler to generate the links
• A new method, VersioncontrolRepository::fetchLogs() that should be the new entry point for all repositories to perform their manual fetches (which will also be useful for cron).

Less related but also rolled in with this are:

• VersioncontrolBackend::loadEntity(), a helper wrapper on VersioncontrolBackend::loadEntities() that will return a single VersioncontrolEntity object.
• Some updates to old ArrayAccess-dependent interacting with VersioncontrolRepository values.