Rework the colored tag feature
    
    This commit does a few thing:
    - Drop the hard-coded list of color
    - Instead of this list, offer the user to pick any color they wishes
    to use in a color-picker widget (html5 powered)
    - Fix editing a tag - simplify the logic while at it so that it runs
    an update query instead of having to manipulate the issues (removing the
    old tag then adding the new one...)
    - Fix updating a tag
    - Fix deleting a tag
    - Small code clean here and there to avoid duplication (most around the
    using the get_tag() method)
    - Adjust pagure.lib for the change in the DB model to use TagColored
    where we were using Tag