Bugfix: Make blankslate-narrow responsive

[crhallberg]

Using max-width instead of width allows the blankslate to shrink on smaller screens.

Fixes: #553.

Sorry for the branch name, I made this change right on GitHub.

/cc @primer/ds-core

[crhallberg]

It's also possible that .container could benefit from the same adjustment: https://github.com/primer/primer/blob/master/modules/primer-layout/lib/container.scss#L4.

The other container flavors are using max-width so it would also be more consistent.

[shawnbot]

Thanks for this, @crhallberg! I'm queueing this up for our v11 release in #498. We need to catch that branch up to some of the minor version work that's been done since we opened it, so don't worry about merge conflicts for now. ✌️

[crhallberg]

Sure thing, @shawnbot! Would it be helpful to make another pull request for the .container width to max-width adjustment I mentioned earlier?

[shawnbot]

@crhallberg Sure, that'd be great. Thanks!

[shawnbot]

FYI, I've decided to include this as an enhancement in v10.10.0 rather than a breaking change. Thanks again!