This is a small QoL change to make the admin easier to do their management job. This makes knowing which row is the correct row to operate easier by simply checking the hover background, especially when they have a wide screen.

Sample:

The 2nd row is hovered, so before the admin clicks the "delete" button, they can knowMagical8bitPlug2 is the one that the "delete" operation will be applied to.

This patch only added the hover background (selectable) to the repo admin and user admin list page. Other tables can be added later.

Note: I'm not sure if mytable.css change is sane, feel free to modify this PR directly if needed/preferred)