Bug 11373: (follow-up) Add "change calculation" feature to the fine payment forms

Bug 11373: (follow-up) Add "change calculation" feature to the fine payment forms

This patch provides some follow-ups, including corrections to the
JavaScript and updates to the text for readability.

- Remove obsolete script "type" attribute
- Update the use of the obsolete jQuery method "bind"
- Use Bootstrap's built-in "show" option instead of simulated click
- Handle preventDoubleFormSubmit function's "waiting" class if modal is
  closed manually

To test, apply the patch and follow the original test plan. Everything
should work as expected.

Note that when the modal is shown, the cursor changes to the "waiting"
one when you hover over the body of the page. If you click "No" in the
modal, the cursor should return to normal.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>