Resolve redundant methods of creating tag implications

[discomrade]

A tag implication (aka. tag parent) is when a tag logically implies another: a fox is always a canine, animal. It is similar to a tag alias (aka. tag sibling) where two tags have the same meaning: smiling is another way of tagging smile.

There are currently two ways to create tag implications:

1. Setting a tag alias as itself plus the implication: fox -> fox canine animal [1], which has recently been officially recognised on the tag alias settings page[2].
2. Enabling the Auto-Tagger extension[3] and adding them.

Both are being used by Shimmie admins and having two ways to tag implications seems like a maintenance and administrative problem. The first is hacky (even if valid) and the second is a misleadingly-named optional extension. I propose they be consolidated into a core extension (like Alias Editor already is), which creates an Implications tab next to Aliases.

I think the best option is to duplicate the aliases table and page, rename and polish it, and then write a SQL migration script to convert any existing Auto-tagger entries to the implications table, and to convert implication-aliases to become implications, and then finally add an input validation check to prevent new aliases being abused as implications. This allows the Auto-tagger extension to be removed.