Add-on Store: Add filter by channel (#14852)
Part of #13985
Summary of the issue:
A user may want to browse add-ons by different channels.
A bug was introduced to the add-on store which caused only one channel version per add-on to be available in the listing.
A user may wish to browse add-ons by channel, as well as view all channels for an add-on.
Description of user facing changes
Added "channel" to the listing columns.
Added a filter by channel.
A bug was fixed where only one channel version per add-on was listed when listing all available add-ons for all channels.
Description of development approach
Add-ons are now collected by channel and add-on ID.
A channel filter was added similar to the status filter.
