Remove entity settings entirely, use configuration to determine what's supported

I'd actually like to tackle this on.

Here's the first step. If I go in and disable the meta tag configuration for "Node" then I should not see any forms on nodes for editing meta tags.

This one changes the logic of metatag_entity_supports_metatags() to look at the config enable/disable state, instead of checking in metatag_metatags_form(). Also fixes metatag_metatag_config_instance_info() to now always return supported contexts.

Tested a bit more logic and scenarios. Next up, removing all the specific entity type and bundle checkboxes/variables that are no longer necessary.

Starting to hack away at the existing variable code, and fixing tests.

Wrong patch, but correct interdiff...

Another try at passing tests.

Trying to work out a caching issue with tests, otherwise this is working as expected.

@DamienMcKenna: Mind taking a look at this? This is how I envisioned "enabling or disabling" support for metatags per entity type originally, just by inspecting the config entities.

Bumping priority. I never envisioned having a wall of checkboxes in addition to the config system. They really need to be unified.

@DamienMcKenna: Can we talk about moving forward with a plan for this?

Fixed merge conflict.

Not sure what to review here. Issue summary is a bit too sparse to understand what the before/after might be for the interface.