Improve meta box message UI [2 hrs.]

[adamsilverstein]

Especially the custom message area could use some polish:

• add a progress or % indicator, perhaps modeled on twitter's, indicate when the entered text is getting close to the limit to help the user avoid going over. I'd love to see some design ideas for this.
• other visual improvements?

[adamsilverstein]

could we do a live preview in gutenberg?

[jeffpaul]

@adamsilverstein is this resolved by #44 (and thus can be closed)?

[jeffpaul]

@johnwatkins0 is this effectively resolved by #44?

[johnwatkins0]

@jeffpaul No, we worked through a good-enough solution for the metabox for 1.0.0, but Oszkar has since provided comps and suggestions in Teamwork (see "Improve meta box message UI" task) which would introduce some somewhat large and very nice improvements. Do you think we should open a new Issue for 1.1.0 improvements, or keep this one?

[jeffpaul]

@johnwatkins0 I'll plan to open new issues for the UX improvements from Oszkar, so I'll go ahead and close this via #44.