Change some uses of `render` to `renderLive`

* `render` uses `serialize`, which can be slow in templates with
  extensive objects in the context.  Specifically, in `tree.mustache`,
  all objects in the list are included in the context, so `render` slows
  down at least linearly.
Check for `GGRC.current_user` before using it

* On initial "welcome" page, GGRC.current_user is undefined, so this
  line causes an error
Ensure valid `template_path` before checking its presence

* If it's not a valid `template_path`, then don't check for `null` or
  `undefined` in `GGRC.Templates`
Merge pull request #894 from reciprocity/feature/render-to-render-live

Various client-side fixes