Add option to mark a feature as "Required: All"

There are several related issues to track down for this. But the issue relates to whether or not to move the "Existing" plugin earlier in the priority chain.

We want Features that have been manually created/edited to keep their assigned config. Right now this is handled by adding the "Required:..." list of required config to the features.yml file.

However, we don't want to duplicate the list of config files from /config/install into the features.yml file like we did in D7's info file.

My proposal is to add an option for marking a Feature as "locked" or "required-all" where any yml file in the config/install directory is required. This would prevent config assigned to the feature from moving to another feature because of a change in assignment plugin. But it wouldn't be as drastic as moving the "Existing" plugin early in the list. Features that have not been marked would still freely allow config to assign to other features as plugins are changed.

I'd propose that a checkbox for this is added to the Edit page for the Feature and is automatically enabled so a Feature exported from the Edit page becomes "locked". Features exported from the main page would not be locked.

The word "locked" is probably not a good one since D7 had the different concept of a "locked feature that prevents reverting". So I'm open to other terminology here.

Marking this as "Major" because it solved several other issues marked as major in perhaps a cleaner way. Also would affect several issues regarding "conflict" and "missing" config status.