Add JSON and TEXT links to more pages about gitiles HOT 5 CLOSED

Ideally, we would offer JSON and TEXT links for every view for which there is JSON or
TEXT available. Doing this manually in the templates is easy enough but is surely the
sort of thing we would forget to do. It might be nice to have it happen automatically,
in which case we would need to teach BaseServlet implementations how to advertise which
formats they support.

FWIW, this would be very helpful for the chrome team.  I recently wrote a tool which
needed to look up commit messages (i.e. from https://chromium.googlesource.com/chromium/blink/+/master).
I ended up writing a regex to parse gitiles html.

Thanks for the suggestion, we are more than happy to prioritize which JSON endpoints
we implement first based on Chrome's priorities :)

That said this bug is specifically about the JSON and TEXT link badges you see at the
bottom of https://chromium.googlesource.com/. Currently we *only* put those links on
/, even though JSON and/or TEXT are available for more pages.

Having json listing for the recent commits, is the only issue (that I'm aware of) preventing
us from moving Blink's "garden-o-matic" tool to a server:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Tools/GardeningServer/scripts/svn-log.js&q=svn-log&sq=package:chromium&l=100

I looked at adding it earlier this summer, then got distracted with other things. 
Eventually I'll get back to it.

re: #3
I don't know if this is new since your comment, but does this work for your json recent
commits listing:
https://chromium.googlesource.com/chromium/blink/+log/HEAD~5..HEAD?format=JSON