Get rid of Addon.latest_version

[diox]

Splitt from #3529

The latest_version field on Addon points to, well, the latest (non-disabled) version. It's used by reviewer tools to figure out which version to review, and internally so set a few things.

With the introduction of version channels, it won't be as useful, since you need to figure out the latest version available in the same channel you're interested in. We could add latest_unlisted_version and latest_listed_version fields, but that's probably overkill and won't scale if we decide to add more channels later.

I want to try to get rid of that field, replacing it with a method that takes a channel and maybe an optional argument to ignore disabled versions. To do this safely, my plan is to do this in 3 steps:

• Keep the field and the methods that sets it, but stop using it directly, instead using latest_version property to emulate it, and modifying reviewer tools queries that directly hit the database
• Then, replace latest_version property with a method using the channel
• Then, stop setting the (now obsolete) field
• Then, remove the column entirely.

[diox]

Filed https://bugzilla.mozilla.org/show_bug.cgi?id=1328917 to let ops handle the column removal.

[eviljeff]

column is gone (on dev, stage, and prod today) so calling it done!