Use topic title, rather than prev author for replies; supporting code

* Add message for this and remove old ones
* Add support for l10n messages to use parameters, on both client and
  server.  Remove caching from server.  The message system has its own
  caching, but caching the full output string at this layer is
  unnecessary and incompatible with parameterization.
* Add debug helper to write template data to console, for client-side
  template debugging (by Shahyar)
* Compile templates

Change-Id: I53a4a36fb91da26d2e156bbe7923609cf8dfdb04
Merge "Use topic title, rather than prev author for replies; supporting code" into frontend-rewrite