Show download links as a tabbed panel

Users can now select which link they prefer to see, and only that
one is displayed in the UI panel.  This reduces the vertical space
we need to show the different command permutations that we support
to download a patch set from a change.

If the user is signed-in, their most recent preference is stored
in the database as part of their user account, so future displays
will default back to that type of link again.

Change-Id: I6bc9c21ad4f9b1e8124530d016aea3c57c2a1bcb
Signed-off-by: Shawn O. Pearce <sop@google.com>
12 files changed