Resolve "Admins can send attachments" [#30]
Notes
Closes #30 (closed) and fulfills its AC's, which were:
GIVEN a relay channel with admins
- WHEN an admin sends an attachment to the channel
- THEN all the members of the channel will receive it
Also does some light refactoring for increased modularity and clearer naming.
NOTE: this MR introduces a hard dependency on JDK 1.8.0 -- which is about two major versions behind, but without which attachment relaying does not work. (Known upstream bug.) In the future we should make this easier to deal with via containerization.