This is intended for features that are experimental enough that we want
to hide them behind some warning text, but not so experimental that we
want them to be enabled only with obscure or undocumented commands.
The two current features, to nobody's surprise, are NoteDb and
PolyGerrit.
Enabling NoteDb sets all NotesMigration options to enabled, in
particular disabling ReviewDb for changes, although the tables are
still created since they're still in the schema.
For PolyGerrit, the main question is what the default UI is, as PG has
been part of the WAR for a while now. It is also possible to disable
the GWT UI, although this is not the suggested default option since
there are still some missing features that make switching back
worthwhile.
Change-Id: I419d73eb5944b7c4c8651b86c6efd25a969e60b5
c561858c13