Cleanup the attachment template, allow separate bug comment/commit body

This makes the first part of "edit attachment" template the attachment name and comment, and text following the "Commit-Message" line to be included as the commit body. Both of these are pre-filled from the commit subject/body for convenience.

Example of new template:

    # Attachment to Bug 11383 - Test bug °!\"§$%&/()=?`^²³{[]}\ß´+*~'#;:_,.-€@©\.|<br><li><img src="http://www.mozilla.org/favicon.ico"/>
    
    # Please edit the attachment description (first line) and comment (other lines).
    # Lines starting with '#' will be ignored.  Delete everything to abort.
    
    Cleanup plugin_focus_helper test events to try and fix intermittent orange
    
    Bug body from git
    
    # Lines following 'Commit-Message:' will be included as the extended commit message
    # in the attached patch. Comment this out to re-use the attachment name/description.
    
    Commit-Message: Bug 11383 - Cleanup plugin_focus_helper test events to try and fix intermittent orange
    
    Bug body from git
    
    #Review: reviewer1@example.com
    #Review: reviewer2@example.com
    #Review: :me+
    
    #Super-Review: superreviewer@example.com
    #Super-Review: :me+
    
    #Feedback: feedbacker@example.com
    #Feedback: :me+
    
    #Obsoletes: 122479 - test
    #Obsoletes: 408420 - u-umlaut test (unix, utf-8)
    #Obsoletes: 541503 - Add bar.
    #Obsoletes: 648949 - Bug 11383 - 780351 - Tests for mozbrowser acting as a window name namespace barrier.
    #Obsoletes: 776817 - Cleanup plugin_focus_helper test events to try and fix intermittent orange
    
    # To obsolete existing patches, uncomment the appropriate lines.