Silently substitute synonyms for tags

[ArtOfCode-]

Instead of showing an error when a user tries to use a synonym instead of the tag it points to, just substitute the synonym for the primary tag.

May assist with #1436, but I'm not sure it's the full answer.

[Oaphi]

LGTM

[trichoplax]

The error described in #1436 relates to editing a tag itself, not editing the tags on a post, so I've added some comments there including more detailed steps to reproduce.

Note that:

• Before this pull request, editing a post to add a tag synonym would already silently replace the synonym with the relevant tag (this was already being done on the front end).
• The error described in Tag synonym already exists error, when removing and re-adding with different capitalisation #1436 still happens after this pull request (it is an error when editing a tag, not editing a post).

[ArtOfCode-]

@trichoplax when I dug into this it wasn't quite being handled on the front end - it would suggest the primary tag instead of the synonym, but it still gives the option to create a new tag. This just cleans up that loose end - anyone selecting the new tag anyway will have it replaced with the synonym's primary.

[trichoplax]

Thanks for explaining - I didn't pick up that when testing it. I just pressed enter after typing in the tag name, and it was automatically replaced, so I overlooked the option to override that behaviour. Good to have this fixed.