Display which version the validator is running on the web UI

[isabelle-dr]

A user who visits https://gtfs-validator.mobilitydata.org/ should be able to see which version of the validator is running.
Ideally, this is not a number we have to manually update but a number taken from the code.

See this comment for more info.

[jcpitre]

I noticed that the rules document does not display the version number either:
https://gtfs-validator.mobilitydata.org/rules.html
This file is auto-generated from the java code. The version number looks like an important information to have when you look at the rules.