Gerrit's CanonicalWebUrl class is already responsible of getting
the full URL from either gerrit.config or the incoming HTTP request.
However, over time, other parts of Gerrit injected directly the
configured string through @CanonicalWebUrl annotation and did not
manage correctly the situation where the configuration is absent.
The only situation where gerrit.canonicalWebUrl is mandatory is
OAuth authentication, because the OAuth provider needs to have
a fixed callback URL with a single hostname.
Change-Id: Idee1d36eb08c53a0d06604b9a737695322e124d1
53eab1080b