Server validation improvements

[tomaszmrugalski]

We're currently (15 May 2021) struggling with station authentication failures when uploading observations. This uncovered several problems that could be improved:

• [ ] when observation reception fails, there is no logging whatsoever. Need to add server logs explaining why the report was rejected
• [ ] the authorize_station does more than just checking authorization, e.g. it does check file types
• [x] the checks are too strict (everything that is not PNG is rejected)
• [x] the station should log the submission result, in particular in case of failure

Regarding the last item, there are some improvements on #47 that improve the station submission logging situation a bit, but the code there still doesn't log the failure - just the 401 error code.

[tomaszmrugalski]

The situation regarding logging has been improved after #63 merge.