Event creation submit button can be spammed

[Xevion]

Multiple events of a single kind kept being created on portal. I identified that it was likely multiple presses of the submit button - and indeed, a simple test on local yielded multiple events being created by the submit button being pressed more than once before a redirect was enacted.

• The submit button should be denounced, or better, stalled until the submission has finished.
• If a redirect is enacted (successful submission), then the submit button should be locked permanently (although, on back button, the submit button should be yielded back to the user).