Add border on Title when focused in order to match other block focus styles

[iamthomasbishop]

When the Title is focused, it doesn't use the same styling (border) as all other blocks. We need to update this to match.

Current State

Title focused:

vs. other type of block:

Proposed

Note: The way the Title currently sits in the editor, the border-top might not be visible. For the sake of simplicity and consistency with all other block types, let's match the styling (border: 1px solid ... on all four sides).

[iamthomasbishop]

@diegoreymendez I can't remember if we discussed already, but if possible (and not already solved as part of this issue, can we also make sure the placeholder text label color (when there is no title) is $gray aka #87a6bc? Thank you!