Show tier name in addition to tier number in messages

[m-i-l]

If someone submits a site that has already been submitted, they see a message such as "The domain {} already has an active tier {} listing with a home page at {}", where the tier is a number, e.g. 1. To be more understandable, it should show the tier name, e.g. Basic in the case of 1.

To implement, update get_active_tier(domain) in add.py to return a tuple (current_active_tier_no, current_active_tier_name) in place of the current current_active_tier. This in turn will involve rewriting sql_select_active_tier in sql.py to do a join on tblTiers to get tier_name, because tblListingStatus just contains tier (which is the number).