Add new icons for embeds, and default block

[jasmussen]

This fixes #203

This PR adds a generic "block" icon, necessary for Gutenberg

In addition to this, it adds a generic "embed" block, as well as a few other topical embed blocks:

• post embed
• audio embed
• photo embed
• video embed

Note, these icons are placed in the "Gutenberg" directory, which means it does not get added to the icon font.

Screenshots:

[folletto]

What I feel works well:

1. The shape and structure
2. The way it locks together type icon and embed icon

What I find not ideal:

1. Video having a different structure and locking
2. Embed generic using the "code" icon, and being different

My advice would be to have the same identical baseline, and reuse it.

This is a crappy mock of what I mean (especially crappy for video):

[field2]

I see what @folletto is saying, and I think it's right. They're a good representation of blocks of code, but should be more consistent–the brackets should be the same size and place, and the "sub-icon" should be reversed out like in audio and post. Having no "sub-icon" in generic is fine. Photo should be enclosed; the subicon shouldn't share a border with the main icon. The video subicon should be enclosed and white.

[jasmussen]

Solid, solid feedback. Will address it monday! Thanks all!

[field2]

Sorry for the cruddy screenshots, but here's a suggestion for unifying the look of these. They're 18px wide now, so it's hard to say if they'd look weird with the other gutenberg embed icons.

[jasmussen]

I dig every one of those, Ben, except the generic one. It feels like the generic one would actually work better without the asterisk — I understand why you did this, but after seeing Davide's empty one I was like, yeah, that works.

[jaymanpandya]

+1 on @jasmussen for the generic icon. 😊 they look awesome. 😍

[field2]

I actually did a generic one without asterisk, but forgot to post. Yep, it's better without it.

[jasmussen]

Solid! Ben, feel free to add your updated SVG files to this branch, overwrite mine. Okay to push. I can grunt/build and rebase if need be. Thanks for the help.

[jasmussen]

This branch got a little messy, so I created a new one and merged your icons with the permission given in Slack.