Add custom links and plugin template content to JobResult detail views

[glennmatthews]

Fixes: #N/A

• Add support for Custom Links to be defined for the JobResult model and rendered in the JobResult detail view and the Job's JobResult detail view (yes, these are two separate views; the former is generic for all JobResults, while the latter adds more Job-specific context for results that were derived from a Job, such as embedding the associated Job's source code)
• Also add plugin_buttons and plugin_full_width_page template tags to both of these views so that plugins can add either of these types of content to these views if desired. (I did not add plugin_left_page and plugin_right_page as these views are full-width views only and don't have left/right columns at this time.)

[lampwins]