Track order

[johanvdw]

This PR close #1305. It gives a position to tracks to make sure their ordering is consistent in dropdowns when entering a CfP and in the online schedule.

I've tested this locally and adjusted a copy of the room unit tests for tracks.
There is no unit test for the order in online schedule.

Screenshots (if appropriate):

(eg, I moved the Rust track to the top)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed: A few tests did not pass locally, but that seems unrelated to these changes
• My change is listed in the CHANGELOG.rst if appropriate.

[rixx]

Thank you, this looks great! Could you look into the failing tests, please?

[rixx]

Thank you so much! I'll make some minor changes after the merge – just to let you know, nothing you were expected to do!

• I'll remove position.help_text, as we never use that field explicitly, and it's more work for the translators. I'll move Room.position.help_text etc at the same time
• For the same reason, I'll change the translation strings to "Move up" and "Move down", as there's no real value in specifying the object type each time and it adds more translations
• I'll add the feature to the changelog