Studio/uses mixed modulestore

https://edx-wiki.atlassian.net/browse/STUD-1540

**MixedModuleStore API**
* Updated Studio and LMS so all callers, with a few exceptions, go through the `MixedModulestore`.
  * to find the exceptions (a few tests and 2 management commands), search for callers to `_get_modulestore_by_type`.
* Updated the `MixedModuleStore` API and all their callers to take in the `user_id` for all Edit operations.
* Improvements to the following APIs:
  - Publish - so it automatically publishes all the downstream children.
  - Delete - so it does the "right" thing depending on which revision is deleted, etc.

**Settings**
* Changed the configuration setting format so the Stores listed in the Mixed configuration can be ordered by preference, via a `list` rather than a `dict`.
* Updated the settings for all Test code so they should all go through Mixed, unless they are explicitly testing a particular store.
* Introduced a new Setting called `MODULESTORE_BRANCH` that specifies the default "branch" (draft or published) to check for relevant modulestore methods.  Currently, the `DraftModuleStore` uses this setting.  Eventually, the `SplitModuleStore` will also honor it.

**Tests**
* Since (most) test code now go through the Draft store, some tests needed to be updated to Publish their newly created items if they assumed they would be in that state.

**Reviewers**: @cahrens (Studio) @dianakhuang (LMS) @feanil (Devops)

Maybe explain this convention? I've seen it other places in our code, but I feel like it's not entirely intuitive.