Bug 28988: Reindent calendar template

Bug 28988: Reindent calendar template

This patch performs general template cleanup to the calendar template:
Make indentation consistent and trim trailing whitespace.

To test, apply the patch and go to Tools -> Calendar.

The page should look correct and work correctly. If you view the diff
while ignoring whitespace the only changes should be where line breaks
were introduced.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>