Google Calendar-esque quick post creation

In google calendar, you can create simple events with just a title just
by clicking on a space in the calendar. I am adding the the same
functionality for post-creation from the Edit Flow calendar.

This was request from our editors at Grist
UI improvements; fixed bug with 'Show x more' functionality

- Users can now double click on a calendar square to open the quickpost
dialogue
- The '+' button is styles to be less intrusive
- The '+' button now only appears after half a second of hovering over
a calendar square
- The '+' button no longer sits on top of post titles in calendar
squares with many posts
- The 'Show x more' functionality for calendar squares with many posts
is working again
Comment Update

This is a comment update, largely to see if my commits are going
through the right account.
Hover style for quickpost button

The quickpost button now darkens on hover.