Extensions used to have access to a siteconfig attribute on

SettingsForm, which was were the settings for the extension lived. The

name was there for historical reasons, due to the form originally being

made for SiteConfiguration. The move to using the new KeyValueForm

in Djblets 1.0 resulted in this attribute being removed, breaking

extensions.
This change restores the attribute, emitting a DeprecationWarning when

accessed. The warning is only emitted once per access per form

instance, in order to avoid spamming the logs.

Tested an extension with Review Board that made use of this. Verified that

it no longer broke.