Improve Bitbucket UI to work with screen reader(BBS-108796)

There are two scenario where, while using screen reader, we are not able to interact with Bitbucket properly:

Scenario 1:

1. Create a new pull request.
2. Move to the edit field to add reviewers.
3. Start typing the first few letters of the name of a reviewer.
4. The list of possible matches is not accessible by arrowing up and down using a screen reader.
5. You need to enter the complete name of the reviewer and then hit enter to get the correct reviewer added.

Scenario 2:

1. Go to an existing pull request
2. Click on ‘More’ and and then ‘Edit’ in order to edit the pull request.
3. There is no way of adding additional reviewers using a screen reader.
There is an edit field but typing the name of a reviewer in the field has no affect.
4. The link for deleting a reviewer is not properly labelled. The link just appears as the ‘#’ character when read with a screen reader.

Test environment:

Latest Chrome and Firefox browsers
Windows 10
Jaws 2019 screen reader