Why this change?
The `Editing sidebar tags navigation allows a user to filter the tag in the modal by selection` system test was flaky
when we were doing `modal.filter("").filter_by_unselected`. The
hypothesis here is that the filtering is debounced before issue a
request to load the new tags and the dropdown is only disabled in the
debounced function. Thereforethere is a chance that when
`modal.filter_by_unselected` runs, it is selecting a row against a
disabled dropdown which results in a noop.
What does this change do?
When filtering using the input in the modal, we will now disabled the
dropdown until the filtering completes which will then re-enable the
dropdown.