Add feature to officially 'obsolete' a document.

[scott-abernethy]

Currently people mark documents as obsolete by various methods, such as changing the name to be prefixed with -obsolete-.

I suggest we add a feature to do this, a button on the document page that will do the following:

• submit a new version of the document
• with title prefixed by "-obsolete- "
• comment stating "Marked as #obsolete" (note that this will add the document to the obsolete hashtag set).
• the operation should require confirmation.

On a document page, if the document is detected as obsolete, a warning should be displayed. (Similar to the out of date version warning).