Taxonomy term field condition

I needed exactly this. The patch in #4 didn't work with 8.x-1.4 for various reasons. The most serious being that the required annotation is now context_definitions not just context. Furthermore, the entity_type_id we most care about is taxonony_term not taxonomy_vocabulary. We also don't need to store vocabulary separately in config - that's what entity_bundle is already for.

While I was digging into getting this working, I decided to refactor NodeField and TermField into a shared EntityFieldConditionBase.php.

This is still a work-in-progress. There are a few @todo comments I've left in the code. It's not totally clean and perfect. But it's now working enough for the project I need, so I'm uploading patches here. 3068125-6.patch is the regular patch created with renames = copies in my ~/.gitconfig. However, composer-patches was having trouble installing that against 8.x-1.4 itself, so 3068125-6.full.patch are the identical changes but with git diff configured to not try to do "smart renames". This seems to apply more reliably, but is a larger patch to review.

Curious what the maintainers think of this direction.

Thanks!
-Derek

p.s. sorry for some of the scope-creep clean-up. But as I'm refactoring everything into a shared base class, it was hard to resist the urge to fix things as I went.