Robust handling of user input

Specifically git url + calendar url string mashing

`base_url = base_url + 'users/' + username`

The issue only points to the `base_url = ...` snippet and mentions git and calendar URLs. Start by locating that code in the repository, then inspect how each URL is supplied and reproduce the reported string-mashing case. Done means the URLs are handled without malformed concatenation and the relevant existing behavior remains intact.

Written by the indexing model from the issue text.