Prevent short-term duplicate thanking

Thanked revision to be stored both server side in the session and client side
in a cookie.  Thanked revisions found in either storage solution will replace
their 'thank' link with the string 'thanked'.
Bug: 46690
Change-Id: I976cd8fbf00856c67b77daeb0d0a952efb371661
Merge "Prevent short-term duplicate thanking"