Merge several outline processors in into UserPartitionOutlineProcessor

[kdmccormick]

From this list of processors, the following processors wrap concepts which, under the hood, are implemented on the user_partitions framework:

• ContentGatingOutlineProcessor
• EnrollmentTrackPartitionGroupsOutlineProcessor
• CohortPartitionGroupsOutlineProcessor
• TeamPartitionGroupsOutlineProcessor (new as of this PR)

Theoretically, they could all be merged into a single UserPartitionOutlineProcessor, which would dedupe some business logic and ensure that any new user_partition schemes automatically work with learning_sequences.

Open questions:

• is this as straightforward as I think it is, or are there leaky abstractions / edge cases we're going to hit?
• performance regressions?
• could the same refactoring be applied to block transformers?

CC @ormsbee