Partial cleanup tooltip does not display time

Context

In Rename the warning message on partial cleanup p... (#354494 - closed) the tooltip was changed to show a time estimation of when cleanup would continue.

Problem

1. The time is not shown on the tooltip
2. The tooltip has a very dark shadow - GlPopover component not displaying in all rows ... (#362646 - closed)

Expected	Actual
...will continue within 24 hours.	...will continue within .

Implementation plan

Summarising comment:

1. modify next_run to next_run_at
2. remove functionality in group context