Custom avatar backends provided by extensions have had some problems

staying enabled or staying as defaults, for a handful of reasons. Some

of these were fixed in Djblets 1.0.1 by preventing the avatar registry

from aggressively disabling backends it couldn't find, but that only

solved one half of the problem.
The other half is that the registry kept its own local state on the

enabled/default avatar backends, which wasn't always correct and wasn't

ever kept in sync with the configuration state changes from other web

servers or processes. Avatar state could load before extensions loaded,

and if the default avatar backend was from the extension, that instance

of the registry would end up seeing it as None. Future threads might get

the right value, however. Even if the configuration was fixed to re-set

the default backend, any existing registries would continue to have the

stale state. On top of this, setting a list of enabled backends would

result in premature saves, which would hit the database more than

necessary and could result in other state issues.
The main fix for this is to remove the local state tracking, and always

use the current SiteConfiguration instead. All methods that look up

state will fetch the current configuration and read from that, ensuring

that if the configuration had recently reloaded, the new state will be

used.
All methods that save configuration will write it to the configuration

as well. This has a couple side effects. First, if some code wrote

configuration without saving (save=False), and then waiting a while to

save, it's possible that the written configuration could be lost (no

different than any other work with SiteConfiguration). Second, if

configuration was written without saving, and the current

SiteConfiguration was then saved manually, the new avatar

configuration would be saved, without ever calling

AvatarServiceRegistry.save(). In practice, this should all be fine,

and is necessary to deal with the state synchronization issues.
Along with this, the enabled_services property has been replaced with

a set_enabled_services() function, so that a save=False can be

passed. That function also no longer does more than one save.
Unit tests have been updated to check the local and in-database settings

for all the mutable operations.

All unit tests for Djblets and Review Board pass.
Played around with the avatar configuration locally. Didn't hit any

immediate issues. Settings seemed to have persisted correctly and

triggered SiteConfiguration reloads.