Changes to support One-Click Revert in Rietveld

Changes to support One-Click Revert in Rietveld. 

* Added a 'Revert' checkbox next to the 'Commit' checkbox. The checkbox is enabled only when an issue is closed. Mousing over the popup displays a message that says that this functionality may not be available for all projects.
* Added revert and reverted_by fields to the Issue data model.
* Can now search for reverted and closed issues.
* Added a new /<issue>/open URL that re-opens the specified issue.
* revert and reverted_by fields are cleared when an issue is re-opened.

Design document is here: http://goto/OneClickRevert (available internally)

Changes are staged here:
https://skia-codereview-staging.appspot.com/12001/


Part of a set of CLs that adds functionality to depot_tools, CQ and Rietveld- 
https://codereview.chromium.org/23618017/ : Changes to support One-Click Revert in depot_tools. 
https://codereview.chromium.org/23483019/ : Changes to support One-Click Revert in Commit Queue. 
https://codereview.appspot.com/13433043/ : Changes to support One-Click Revert in Chromium Rietveld. 


Tested with:
* Unit tests have been added.
* I also tested end-to-end using a skiabot-test repository in https://skia.googlesource.com/ (it is hidden). CLs I tested with are:
** https://skia-codereview-staging.appspot.com/12001/ : Adding file4.
** https://skia-codereview-staging.appspot.com/18001/ : Deleting 2 files. Adding 1 binary file. Modifying 1 file.