The current naive approach at retrying is not exactly gentle: if the
server is overloaded we retry right ahead making the problem just worse.

See https://encore.dev/blog/retries

 for a fun explanation of how the
naive approach can be catastrophic.

Fortunately the `tenacity` library allow us to add a bounded exponential
backoff while also making the code easier to understand.

This should make the dashboard behave much better toward GitLab and OBS
when they hit some issue.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>