Make it easy to view case activity attachment

Our user finds it difficult to know which case activities have attachments when looking at the manage case screen. I have made this easier for her by adding a 'view attachment' link where an attachment has one activity.

I had hoped to also add an icon column to indicate where activities have one or more attachments (above only works for exactly one attachment) but although I could add a column to the JSON I couldn't figure out where to change the column headers so will log this in case someone else wants to pick it up & take it further.

https://github.com/fuzionnz/civicrm/commit/0bb65ff8c28b905148c5a127349cd4f71afdcbf7