Improve accessibility for Bitbucket's pull request page

On Bitbucket's pull requests page, the drop-downs for pull request status, author, and target branch are not labeled to indicate they are drop downs and are considered as text fields for accessibility software such as Apple's VoiceOver.

The same drop-down items cannot be navigated using the keyboard arrow keys or VoiceOver's navigation shortcuts, for example, as there is also no audio announcement of the values.