Add ETag to documents

[n-dusan]

End goal is to make the user do a simple reload and retrieve the latest changes on a web-page. Stelae will address both current and historical pages getting cached.

• Add an ETag on current and historical documents by taking first part of the commit hash (or maybe, better, blob hash?) for that document

advantage of blob hash - as long as the document doesn't change, the blob hash doesn't change, even if there are dozens of commits since the last time the file changed.
advantage of commit hash - it's really easy to get, and we are familiar with it.

I'd investigate how easy it would be to grab the blob hash - probably as we are traversing the git tree.