snap: Add initial build configuration for Snapcraft

Michael, you said you were interested in exploring a Snap build of Epiphany so here you go.

Because I didn't want to build webkit from source now yet, this is a 3.30 version. It builds with 18.04 LTS's webkit2gtk and gstreamer packages.

It looks like we have a sandbox issue so I had to build this first version using devmode to avoid these errors and allow it to actually show webpages:

Failed to create shared memory file /WK2SharedMemory.3901413086: Permission denied

I pushed the snap to my server so you can download and then install with these commands on Ubuntu or a system that supports snap:

$ snap install --dangerous --devmode epiphany_3.30.2_amd64.snap

And then assuming you don't have another version of Epiphany running:

$ snap run epiphany

If you like what we've got so far, I can start the process of getting this published as an edge snap on the Snap Store. An edge snap isn't visible in the search results but allows people who really want it to install it.