No dirty form warning when filling out a pull request (BB-5090)

Steps to reproduce:

• Click 'Create pull request' button
• Fill out the Title / description
• Accidentally press 'create pull request' button again instead of 'Send pull request'

Actual results:

• The form is cleared

Expected results:

• A warning that you're going to lose your work (or disable the 'create pull request' button when you're on the create pull request form