Unlisted choice improvements

[batpad]

Towards #756 and #757 -

• Allows for an other_text option inside unlisted_choice which allows user to specify a custom text in the select drop-down
• Hide select drop-down and label if no choices are defined
• Handles no choices in backend processing

I might have underestimated the complexity in hiding the select dropdown if there are no choices present, and that bit seems to have gotten a bit ugly. Will leave some comments in the code to see if there's things we can improve.

@yuvipanda @consideRatio @GeorgianaElena - firstly, thank you so much for fixing up issues with the unlisted_choices work, and many apologies for the delay in getting to these fixes. I am not super happy with the implementation in this PR, and I'll leave some more notes with the code. I did a bit of testing though, and it seems to me like it all works as expected.

If this approach seems ok to you all, I think what might be pending:

• Create an example profile option to not have any choices but have an unlisted_choice to make testing easier
• Create a test with no choices to make sure the backend handling is covered with tests.

[batpad]

Oops, my bad - I thought I was up-to-date with kubespawner:main but apparently not. Just going to close with PR and open a new one cleaned up. Apologies for the noise.