Replace all occurances of 'studio' with 'cms'

[kdmccormick]

Context

Since edx-platform ADR-13 was added, it's agreed the "cms" is the correct name of the webservice, whereas "studio" should only be used in a product context.

However, Devstack currently calls the service "studio". For example, to start CMS, Devstack users must run: make dev.up.studio; to attach to CMS, Devstack users can run: make studio-attach.

It may reduce confusion for Devstack users if the "studio" service (which is defined in docker-compose.yml, and referenced in docker-compose.*.yml, the Makefile, and other locations) were renamed to "cms". That would mean that users would run instead make dev.up.cms, make cms-attach, etc.

Acceptance criteria

(this is my best guess; there may be missing steps)

• In docker-compose.yml, rename the "studio" service to "cms".
• Search the entire devstack repository for "studio". Wherever "studio" is a reference to the docker-compose service, rename it to "cms".
• Update the docs. For example, references to make studio-shell should be changed to make cms-shell.
• Make a forum announcement about the change. In the announcement, recommend that users:
  • first stop their platform: make down
  • then, pull new devstack code: git pull
  • then, confirm that they can start CMS: make dev.up.cms