REST: Resolve performance issues with tags

REST: Resolve performance issues with tags

The list comprehension used to generate a list of tags resulted in a
significant number of duplicated queries. Resolve this by copying the
approach taken to minimize patch queries in the UI.

This changes the output of the response in two ways. First, counts for
all tag patches are now shown, even if the count is 0. Secondly, a
dictionary is returned, with the tag name as the key, rather than a
list. As such, the output for a typical patch is transformed from:

    [
      {
        'name': 'Reviewed-by',
        'count': 1
      }
    ]

to

    {
      'Reviewed-by': 1
      'Acked-by': 0,
      'Tested-by': 0
    }

How this is achieved is a little hacky, but reworked tags are on the
post-v2.0 which will allow this to be resolved.

Signed-off-by: Stephen Finucane <stephen@that.guru>