Add joined rooms & pending invites to client page

[s3lph]

This PR adds joined rooms and pending invites to the "client" page in the Maubot Management interface:

• Already joined rooms can be left
• Pending invites can be accepted, ignored or rejected

The motivation behind this is that AFAICT, when autojoin is disabled, it is currently impossible to manage room memberships without using a second Matrix client.

This is the first time I'm doing something with any of React.js/JSX or SQLAlchemy/Alembic, so please feel free to let me know when something appears odd.

There certainly is room for improvement (e.g. I'm not entirely happy with the way I'm interacting with DBInvite all across maubot/client.py, but I haven't yet come up with a better solution).

[wolfspyre]

what would it take to get something like this merged in? seems like a good starting point to be able to address encryption-key validation requests?