Explicit label and help text for TTLs

https://redmine.teklia.com/issues/10971

The TTL shown on each task in the process status view should be preceded by Time limit:, and followed by a help icon. Hovering over the time limit shows a title that reads:

This task has a maximum execution time set. Once this time is reached, the task will be cancelled.

When there is no TTL on the task, it can be hidden completely instead of showing a slash and an infinity symbol.