Add updateBitsBranchPointers script to create/update 'current' / 'stable' symlinks

Bits static asset URLs currently reference a specific production branch. This
is fine in most cases, but in certain cases (such as large files in extensions
that change infrequently) we want a stable URL to the latest version of some
asset, so that the referenced asset is not thrown out of the browser cache on
each release. The current case is the UniversalLanguageSelector extension,
which serves large font files out of its repository.

This patch adds a new script to multiversion/, updateBitsBranchPointers. The
script updates a pair of directories in the Bits docroot, static-current &
static-stable, that contain symlinks to the asset paths of the most recent &
secondmost recent production branches, respectively. If only one production
branch exists, both current & stable pointers are pointed to it.

Bug: 55506
Change-Id: If597d70625cf8f06c0e59dd7749442ba05769174