JSON Snippets

Here is the WIP for snippets for android. 

Notes:

* url: I chose an CharField instead of a URLField in case we want to use special urls like "about:config"

* icon: Since now fields are not dynamic we could to the icon to Data URL conversion on the server, but that would require PIL. I re-purposed the template code to do it FileReader API.

* frontend: The frontend listing active snippets needs update. maybe we can do this as part of another bug.

* admin and views: I tried to re-use as much of the current codebase, by moving shared code to common 'base' classes and views wherever that made sense.

* Snippet weights: Do we want support for that?

This still needs tests and maybe some tweaks here and there. I'm pull requesting this so I can get some feedback on the (refactoring) decisions.

I tested that with Margaret's custom Fennec build and it works! :D