Improve usability for the visually impaired users

Summary

A user being disabled and in need of a braille device, noticed that Bamboo but also Bitbucket Server aren't usable for him.

Suggestion

Specially the following points have been noticed:

• the diff of a pull request isn't accessible / readable
• upon creating a new branch, it's impossible choosing the branch type because the corresponding dialog box isn't accessible
• logging out isn't possible