Add ability to filter snippets by their template.
Stop hiding Client Match Rules on the snippet admin page.

Because Every time I open that section I wish I didn’t have to.
Merge pull request #74 from Osmose/stop-hiding-cmrs

Stop hiding Client Match Rules on the snippet admin page.
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.