Move jinja2 templates to .html.jinja2 extensions

[jace]

Following from #103, we need to rename all templates to .html.jinja2 extensions.

However, several templates including baseframe.html, layout.html, baseframe/forms.html and others are referred to from client apps, so renaming them will break clients. We have three options:

1. Don't rename them.
2. Rename but place symlinks at the old names (will possibly break on Windows-based dev environments, apart from creating chaos on Dropbox and other cloud-synced developer filesystem environments).
3. Create shadow templates at the old names that simply consist of {% extends "<template>.html.jinja2" -%}. This will work for layout templates but not macro templates.

Approach 2 seems the best at this time.

[jace]

Approach 3 may be more sensible. Layout templates can use the {% extends %} directive, while macro templates can use {% from "<template>.html.jinja2" import * %} (or explicit names if * is unsupported).

A deprecation alert comment should be included at the top:

{# DEPRECATED: This template is due to be be removed. Use the referenced template directly. #}