MNT: Convert cxi to submodule

[ZryletTC]

Change of approach from #1
First step of splitting this repo into multiple smaller repos. Replacing each subdirectory with a git submodule linking to the new repositories. This is done following these instructions: https://coderwall.com/p/a3a5xg/splitting-a-project-sub-directory-to-a-new-git-repo

The new cxi repo is here: https://github.com/pcdshub/cxi-config
When this PR is merged, I'll pull it into /cds/group/pcds/config and init the submodule there.

If all goes well, this process will be repeated for all other hutches as well (and probably the non-hutches too?)

Currently testing...
I was concerned this might disrupt iocmanager's commits but it looks like it doesn't do any git pushes so it shouldn't care.

[klauer]

(More context in #1)
I think that before we go any further with this, we should decide if cxi-config and even epics-config are the appropriate names here.

Ideally, these would be specific enough to know what's in the configuration repository and disambiguate them the hutch-python-specific config which is a bare "cxi" https://github.com/pcdshub/cxi/

It may not seem a big deal now, but this is likely to cause confusion down the line once we have 10 hutch and 10 hutch-config repos...

[ZryletTC]

I agree, not sure on good names though. Something like apps-config-cxi?

[mcb64]

I like the sound of apps-config-HUTCH.