Remove redundant 'Prev' and 'Next' string constants

The 'Prev' and 'Next' strings used in paginated lists of groups
and projects are the same, but are defined in separate constants.

Replace them with constants that are used by both.

Change-Id: I88e6bc0a9553c871eac408aca216dd223217fa28
Support writing notedb data into a separate set of repositories

We want adventurous server admins to be able to experimentally turn on
writes for notedb data soon, long before notedb is ready for
production use. However, there are some performance concerns, such as:

 - Doubling the number of refs in every repo.
 - Increasing GC cost.
 - Reduced object locality within packs.
 - Creating lots of garbage (including loose objects) as the notedb is
   regenerated.

Some of these performance issues we will need to address in JGit (e.g.
by modifying the repacker), and some might not turn out to be so bad.

For improved isolation during this transitional period, allow for
storing all metadata in a separate repository from the main one. This
can have a separate repack schedule and can be safely blown away as
necessary.

In the local disk case, metadata repositories are stored by default in
$site_path/notedb. We are not committing to this location long-term,
and there is an undocumented configuration option that may change.
Adventurous admins are responsible for rebuilding the notedb and
cleaning up garbage if this configuration changes.

Change-Id: I8158e3f46eb8378ca1726cf6bad67efa669b40d9
Reduce number of valid NotesMigration permutations

Having one boolean per reviewdb table being replaced is overkill; in
practice, even during the migration, even on googlesource.com, we are
never going to turn on reading from some Change sub-tables but not
others. However, we will very likely turn on writes for changes before
writes for, say, accounts, so we do need some granularity.

Explain the supported level of granularity in the NotesMigration
javadoc.

Change-Id: I98261fa888b3f6b71ed896e6c791aeee8324e898
Merge "Remove redundant 'Prev' and 'Next' string constants"