Create new digest email for forum moderators

Depends on edx/edx-platform#3676

This PR creates a new digest email for forum moderators, providing a list of flagged posts in the moderators' courses.

Note: This feature is configured to be disabled, by default.

Dependencies: This command grabs a list of flagged posts by running a
Rake command (currently via Heroku).  This list can alternatively be specified by manually providing an input file. The list of moderators responsible for each course is retrieved via a new endpoint for the `user_api`.

This commit creates a new `forums_digest_flagged` management command.
This can be used to manually trigger the digest.

This feature can be scheduled to run at regular intervals, though it is
disabled by default. It can be enabled by specifying a positive
value for `FLAGGED_FORUM_DIGEST_TASK_INTERVAL` (default=0/off).
This value should be set to `1440` to be run daily at midnight.

The command used to grab the list of posts can be specified via
`COMMAND_FETCH_FLAGGED_FORUM_POSTS`, e.g.,
Heroku
    `heroku run rake flags:flagged --app=APP_NAME_HERE`
or directly on the server
    `bundle exec rake flags:flagged`
or locally
    `cat ./MY_LOCAL_TEST_FILE`

Accompanying tests have been provided for all affected code.