Bug 24610: Let user switch between 'Pay' and 'Write off' mode

Bug 24610: Let user switch between 'Pay' and 'Write off' mode

In patron's accounting section, under 'Make a payment' tab, you have the
choice between 'Pay' and 'Write off'. It takes you to another form, but
once there you cannot change mode. You have to click on Cancel,
re-select the lines you had selected (if you made a selection) and click
on the right button.

This patch adds link above the form to easily switch between the two
modes.

Also fixes a CSS bug to be able to use Bootstrap's nav pills inside
.statictabs

Test plan:
1. Create some manual invoices
2. Go to 'Make a payment' tab
3. Click on the 'Pay' button in a table row
4. Above the form you should see two links ('Pay' and 'Write off').
   Click on 'Write off', confirm the write off and verify that it did
   make a 'write off'.
5. Go to 'Make a payment' tab
6. Click on the 'Write off' button in a table row
7. Click 'Pay', confirm the pay and verify that it did make a 'pay'
8. Go to 'Make a payment' tab
9. Select some lines and click on 'Pay selected'
10. Click on 'Write off', confirm the 'write off' and verify that it did
    make a 'write off'
11. Go to 'Make a payment' tab
12. Select some lines and click on 'Write off selected'
13. Click on 'Pay', confirm the pay and verify that it did make a 'pay'

Signed-off-by: Christofer <christofer.zorn@ajaxlibrary.ca>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>