Limit archives to only enabled formats

Refactor the configuration of enabled archives to be known to the
GetArchive REST API handler so it can allow only formats that have
been enabled by the administrator.

Use the order listed in download.archive to configure the web
interface, with no formats enabled if the list is empty.

Default to all archive formats enabled, matching the schemes.

Change-Id: I29436f13bbc6fd4a20a7d9151e7f47186f5fbf4b
7 files changed