browse: Remove duplicated releases in snapshot content dropdown

As multiple snapshot branches can target the same release, multiple
occurrences of the same release could be found in the snapshot content
dropdown located in the top left part of the browse Web UI.

So ensure to remove duplicates by using the ifchanged template tag in
related django template.

As a consequence snapshot releases are now sorted by descending order of
their names and are displayed in the same order in the different parts of
the Web UI.