Record MAVLink telemetry in a `.tlog` file

This is generally expected functionality for control station software, and provides some data security in the case of a lost vehicle.

Not sure how hard it is to continuously write to a file from a web-app, but hopefully it’s possible and not too problematic.

The issue names MAVLink telemetry and `.tlog` recording but no files or tests. Start by locating the telemetry-handling entry point and checking the expected `.tlog` format; done should include continuous recording during a session and a usable file after the connection ends.

Written by the indexing model from the issue text.