Clear Add Donation Form After Pressing Submit

[AdrianAndersen]

Currently, when pressing submit in the create donation form, the input fields are not cleared immediately, and there is nothing stopping you from pressing submit again. This results in duplicate donations, which is really bad for the donation statistics.

To fix this, disable the button after submission, and clear the input fields immediately.

Here is an example of how the data will look if the duplicates are not handled: