Description of problem:

When using a vertical monitor, some of the custom migration targets are off the viewport and because there is no horizontal scroll-bar it is not possible to reach them.

Version-Release number of selected component (if applicable): MTA 6.1.0

How reproducible: Always

Steps to Reproduce:
1. Log into MTA and go to Administration -> Custom migration targets
2. Horizontally reduce the size of the browser window

Actual results:

Some custom migration targets stay off the viewport making it impossible to reach them

Expected results:

CSS Grid columns should be different for different screen sizes or a horizontal scroll bar should be displayed

Additional info: