Add button to insert blocks.

Behavior

A plus shows up at the end of the post. Click it to invoke the inserter:

When you make a linebreak, the plus moves to the left:

This is also how you would insert content between two paragraphs:

You can also type / on a newline to invoke the inserter, just like how it works in Slack and many chat apps:

There will likely be a permanently visible "Insert" button in the toolbar:

See also #34 and #72 (comment) for alternate mockups for improving the look of the inserter.