Allow custom text when jobs are held

When a job is held, the build log will contain the text "Outside working hours - job will run during next available time.".

It would be nice to have the ability to specify a different string in the UI as an option, and use that if provided.

Start by locating the code that adds the “Outside working hours - job will run during next available time.” text and the UI configuration for held jobs. Add an optional custom string, use it when provided, and verify that the existing default remains unchanged when no custom text is set.

Written by the indexing model from the issue text.