Specify allowed archive formats in gitiles.config

With the default compression options, some archive formats may be
CPU-intensive enough to slow down a server, so give administrators the
option to disable them.

Formats are specified as a whitelist of archive.format options,
defaulting to everything (except zip, which is still not supported for
security reasons). A link is only shown in the commit detail template
for the first specified format.

Since archive formats are now needed outside of ArchiveServlet, move
the enum to its own file.

Change-Id: I8d5a66678aae75f2bbe54ef4883ca4a070b96817
10 files changed