Chrome: Fix empty post placeholder hover effect

[youknowriad]

Addresses the comment here #1195 (comment)

The input has not before content area which caused a regression in the hover effect.

[aduth]

Minor: Adding the border pushes the text to the right by 2px. I only noticed it upon refreshing from master to this branch, but it also appears slightly offset from both the title and following inserter.

We might consider a margin: 0 -2px to offset (optionally applying both margin and border only ever to the left side).

Can't do margin because we need the auto. Since we're already applying position: relative, we could do left: -2px instead.

[aduth]

While we're here, you should throw a readOnly on that input, because it's logging an error:

react-dom.e5cfcfc9.js?ver=4.8-RC2-40880:1149 Warning: Failed form propType: You provided a value prop to a form field without an onChange handler. This will render a read-only field. If the field should be mutable use defaultValue. Otherwise, set either onChange or readOnly.

[youknowriad]

Feedback addressed, merging to use this as a base for #1209