[ENT-660] Assume that edx-enterprise is deployed in the LMS runtime

*Description:* This PR removes all LMS REST API implementations that could just as well be Enterprise DB calls. It replaces them with some useful interface that comes from the LMS instead. If no such interface exists, we can just directly make database calls. The idea is to assume we are sitting in the LMS runtime, but since we're still not officially a codebase embedded in it, we maintain a clear interface through the use of API clients (just not REST ones).

*JIRA:* https://openedx.atlassian.net/browse/ENT-660

*Dependencies:* A corresponding PR in the LMS is also required to meet the above assumption across the two parties, although it is not necessarily a physical dependency to this PR, and vice versa. Will link it here anyway when that gets started.

*Merge deadline:* June 4th '18.

*Testing instructions:*

Since a lot of places are affected by this, it'll be necessary to test a lot of different Enterprise functionality.

Actual instructions TBD.