Allow use of snippet_id in template variables.

Now the snippet ID can be included in either a snippet template or the
template data stored on a snippet. Mostly useful for Raw code snippets
that still want to access the snippet ID.
Only hesitation I had here was that the spaces are significant here; in Jinja2 you can just do `{{snippet_id}}` or `{{ snippet_id}}` or `{{snippet_id }}` or `{{ snippet_id }}` and all of them work. I figured using different braces and removing the spaces was a decent signal that this isn't opening up all of Jinja2 for you.

Perhaps going further and using a different kind of brace, like `<snippet_id>`?