Improve Select Menu spacing

[simurai]

This makes the check icons optional. Which means that if no check icons are added, the left padding is reduced and aligned with the title:

Before	After

Closes #843

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[simurai]

One side effect of adding the icon as inline block is that there could be a "white space gap", depending how the markup is authored. Also the Primer docs remove the white space, but storybook doesn't.

There are a few options to remove it all the time.

[simurai]

there could be a "white space gap"

SelectMenu-items now use flexbox to align all their content. And can always be changed by adding d-block if needed.

Ok, I think this is now ready. /cc @shawnbot

[shawnbot]

Just curious: why 16px here instead of $spacer-3?

[simurai]

My thinking was that the fixed width has less to do with "spacing" and is more related to the maximal width of Octicons, which are 16px.

But yeah, I'll change it to $spacer-3 to avoid ✨ magic numbers.

[shawnbot]

I had one question about the use of 16px, but otherwise this LGTM. Thanks for tidying this up!