Update FCM specs to match new FCM styles

[niqjohnson]

Update the FCM documentation to match the new FCM styles that went into production a few weeks ago.

Additions

• Added specs for play button on video FCMs

Removals

• Removed specs for the call to action button, which is now no longer an option in FCMs
• Removed all mention of and specs for the category slugs

Changes

• Updated visual spcs
• Updated character counts
• Updated FCM screenshots at all three sizes

Testing

If you have the Design Manual running locally, pull this branch and visit the FCM page under "Page components". If you don't, take a look at the screenshot below.

When reviewing, please pay particular attention to:

• The updated visual style specs. I'm pretty sure I have all the typography, color, and size specs matching the new FCM, but I was pulling them from a lot of sources, so some extra eyes on those would be appreciated.
• The description of how the image size and aspect ratio changes with screen size. I'm not 100% sure if how I'm describing it here makes sense.
• The updated character counts to make sure they're what we agreed on.

Screenshots

Todos

• Once this is approved and merged, I'll update all the other mentions of the FCM in our documentation to match these specs.

[niqjohnson]

Thanks for taking a look, @huetingj and @Scotchester. I pushed an update that incorporates your feedback:

Let me know if you see any other changes.