Add ability to filter snippets by their template.
Merge pull request #70 from Osmose/filter-by-template

Add ability to filter snippets by their template.
Add the ability to use a checkbox for a boolean template variable.
Remove instant preview from the snippet admin form.

The instant preview has a few issues:

- Occasional errors due to weird iframe back-forward behavior.
- Slowness in editing large snippets due to constant sending of snippet
  data, as well as from snippets that take up some CPU.
- Restrictions on functionality due to being in an iframe.

Thus I’ve removed the iframe and the instant preview and added a button
to trigger a preview in a new tab.
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.
Upgrade factoryboy to version 2.4.1.
[fix bug 1082208] Support media files and CDN.