Install synadm into synapse app image

[infogulch]

I need to do some maintenance on my matrix server, and found this cli application, synadm:

https://github.com/JOJ0/synadm

Command line admin tool for Synapse (Matrix reference homeserver)

I think it would be neat if it was installed and configured in the Matrix/synapse server image by default, to enable easy maintenance by just opening the app's terminal.