MP6: Install Plugins table overflowing

[johnbillion]

The plugin listing table used on the Plugins -> Add New screen overflows the side of the screen on narrower devices, cutting off the text for the descriptions.

We could do something similar here to what we do on the Plugins screen itself, and force the columns into rows.

[Joen]

Added patch that restyles the "add plugin" page to be responsive, inspired by the similarly responsive plugins page. Looks like this with the patch:

Notes: some merge-fu will be required as this is committed, since 25977 patches the same file.

[iammattthomas]

In 26223:

Make the Install Plugins search results page responsive, so the table doesn't overflow the viewport. Props joen, fixes #25976.

[SergeyBiryukov]

In 26240:

Fix CSS formatting. Remove redundant rule. see #25976. see #25824.