Button disabeling when inporting form Bank

[agmes4]

Motivation

for importing Banktransactions we should not be able to click the button more then once. The button should be disabling it self instate making sure the user cant send a second request when the last is still on going. Also for user feedback it would make a huge difference.

Hints

Request is send via form submit there should be another action to disable the button after the first click

[Crasher508]

https://github.com/agdsn/pycroft/pull/778