Taxonomy cache fixes

[jasonvarga]

This fixes a handful of taxonomy + stache + cache related issues:

• Terms not deleting when you try to delete them Taxonomy entries will still be shown after they have been deleted #1349
• Terms showing the slug instead of the title New taxonomy requires manual cache refresh to show title #1982
• An undefined offset 1 error when creating terms Term::make() inconsistently results in ErrorException Undefined offset: 1 #2020
• Entry keeping term association when you remove it. Removing Taxonomy from Entry and save doesn't remove from cache. #1870
• Terms showing their id instead of the title when you create new ones on the entry edit page.

This is a combination of PRs #2684 and #2685

Fixes #1982
Fixes #1349
Fixes #2020
Fixes #1870

[johncarter-]

Thanks so much, Jason.