[lib] Display a dev prefix for dev invite links

Summary:
This is an approximate solution. It won't work if someone decides to e.g. serve their keyserver from a different port / url. But still, it is an improvement, which will work for most of the cases, will simplify the testing, and will significantly decrease the number of false alarms regarding links not working.

https://linear.app/comm/issue/ENG-7319/display-dev-prefix-on-dev

Test Plan: Check if a link is displayed correctly on dev web.

Reviewers: inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11839