Update GitHub Gist block so that the URL can be updated

[kellenmace]

Currently, a URL can be entered only once and there isn't a way to change it. We should provide a field for updating it.

[kellenmace]

I have updated this block with controls for updating the gist URL after it's been initially set. I could use a little help styling the block controls, though – at some browser widths it is either too wide, or its bottom border collides with the border of the block below it. Example:

When the viewport is big enough, I'd like to make the block controls URL input field at least around 350px wide, so you can at least see the beginning of the gist number in the URL.

@jomurgel – is this something you'd be willing to take a look at? I added a new editor.scss file to house the block control styles.

Thanks.

[ImBigWill]

I did some work in this branch for this block late last week/early this week. I'll need to double check some of it, as I did some other work for back end styling of blocks that might be redundant here, so I'll just need to double check that first thing tomorrow.

[jomurgel]

Made a couple of changes here and confirmed everything working on this, so I think we can issue a PR for the block updates, though I'm not 100% sure if this is all that was needed — styled form element and the ability to update which @ImBigWill and @kellenmace tackled already.