Bug 37730: Batch patron modification header scroll

Bug 37730: Batch patron modification header scroll

When scroling down <staff_url>/cgi-bin/koha/tools/modborrowers.pl,
there is a point where the table header leaves the normal flow of
the page to stick to the top of the screen instead. When this
occurs, the header becomes fixed: if scrolling through the table
horizontally, only the body will move. This is an issue as it means
that in this situation, some column headers cannot be viewed and the
data does not necessarily match the header under which it is found.

This patch resolves this issue by allowing the header to scroll
horizontally and ensuring that the width of the column headers
still match that of the columns themselves.

Test plan:

A) Reproduce the bug as per Pedro's intructions:

***
1) Visit "batch patron modification":
<staff_url>/cgi-bin/koha/tools/modborrowers.pl
2) Enter multiple cardnumbers, in k-t-d you may use these:
23529000104159
23529000120056
23529000714163
23529000651225
23529000445172
23529000105040
23529000224510
23529000591678
3) Press "Continue"
4) Scroll down the page enough that the table headers row becomes
   "detached"
   and sticks to the top of the page as you scroll down.
5) Scroll the table horizontally. Notice the headers are stuck even
   though
   the data columns have moved horizontally, causing the headers and
data to
   not match.
***

B) Apply the patch
C) Go through steps 1-5 again. This time, notice that the header scrolls
   along with the table body and that the column headers and data always
match.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>