feat: Add support for the Communications MFE

[arbrandes]

The Communications MFE is usable by instructors to send bulk email to learners in a course.

The feature itself (the ability to send bulk email) pre-dates this MFE, and must be enabled as usual for the "Email" tab to become visible in the Instructor Dashboard. The difference is that with this change, the tab will include a link to the MFE by default.

Currently depends on:

• [x] https://github.com/openedx/frontend-app-communications/pull/125
• [x] https://github.com/openedx/frontend-app-communications/issues/126