Fix description length validation and tag limiting

[askmeaboutlo0m]

NOTE: This contains the stuff from #305, since I don't want to put work into fixing conflicts twice from first cherry-picking it out and then rebasing it again later. Either merge that other PR first or do only this one. Look at the individual commits to get a clean diff. Resolved.

The validation functions would get passed the raw description as entered by the user and would make some half-hearted effort to parse them into something sensible, but this completely breaks down in the face of shortcuts like {default}, username shortcuts that get turned into long links etc. Now those functions get the actually parsed description passed to them, the only unresolved shortcut in them being the {tags} one, since the length of that is variable and up to the validation function to deal with and tell the user if tags didn't fit.

Since #286, tags insertion is also pretty much busted entirely, since it runs at the wrong time, so it may insert too many tags and then cause an error down the line because the resulting description is too long. That insertion logic has now been moved to the end of the description processing, where it belongs.

[mvdicarlo]

I kind of want to re-evaluate tag insertions into descriptions.
I've slowly been souring on content injection through custom shortcuts that depend on other fields because it increases complexity. These might be better choices as checkboxes to append or prepend text. It may be more limiting but easier to work with.

[askmeaboutlo0m]

I preferred the old approach of just appending tags anyway, it worked fine. I don't think that's directly related to this patch though, since the description length counting previously got all shortcuts wrong, {tags} is just a particularly egregious case.

[askmeaboutlo0m]

Just ran into an issue where it somehow ended up miscounting the tags regardless. Turned this into a draft for now until I can figure that one out.