UX: post to blog vs. community

[roadscape]

User story: https://steemit.com/hive-144703/@connecteconomy/i-keep-posting-in-the-wrong-place-by-mistake

We can improve this: when posting into a community, show a banner/callout clarifying the behavior ("Posting to community: Communities Feedback"). Include a button to "[Post to blog instead]". When posting to blog and there is a community in the "undo" state, include a subtle option to go back to community-posting mode.

More robust solution would involve being able to save multiple drafts; it's our current handling of drafts that makes this behavior a bit awkward.