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Backport #16576

This PR restores the vendored and patched dropdow from #8638. It
however, it partially abandons the call to `click()` using instead the
default dropdown click calls instead. This prevents the issue of the
dropdown grabbing focus permanently however, this may have negative
effects on the effect of focus on the dropdowns.

Of note, the behaviour of the template selector dropdown on the repo
creation page is slightly odd - I don't believe that this odd behaviour
is caused by this PR but rather by the feed source for this. I suspect
that the dropdown should be adding a delete button to its selection.

Fix #15172
References: #7057
Signed-off-by: Andrew Thornton <art27@cantab.net>