Add BUTTON_ language strings for post & PM buttons

Post/PM body buttons for functions like "edit", "delete", "report" etc. use the full description language strings in both the 'title' attribute as well as the content of the hyperlink itself.

When a custom style doesn't use images for these buttons, they become unnecessarily long due to the descriptive nature of the strings.

I suggest using the existing strings for the 'title' attribute, but using short-term button language strings such as "Edit", "Delete", "Report" for the contents of the <a> elements. Some such strings are already present:

• BUTTON_EDIT
• BUTTON_QUOTE

however others such as BUTTON_DELETE and BUTTON_REPORT are missing.