Ability to 'lock' option value to only autocompleted values

[Chiggy-Playz]

It would be cool if chat input command args which have autocomplete could take in another field called autocomplete_only which can be false by default. This would force the user to use something from the autocompleted options only and not be able to send their own input. This is (was?) a bug on an android build and I think it would make for a good feature!

[A7mooz]

This is still a bug in phone even in beta

[onerandomusername]

This would honestly be a great feature. It saves the additional step of validation locally upon submit, which is quite nice. The validation has already been run, as well, in order to return autocomplete results.

[elkowar]

This would be a neat alternative to #3510 -- having both would be amazing ^^