Update new merge request form to match Pajamas specs

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Problem to solve

In #204818 (comment 601169638), we identified a number of areas of the UI that don't follow the Pajamas guidelines about form validation. According to Pajamas, all fields should be required unless specified with an (optional) and we shouldn't disable the Submit button if a required field is blank. However, the merge request page doesn't match these specs.

Solution

• Keep Create Merge Request button active (not disabled)

• Implement an error message if the user hasn't completed a required field