Enable multiline chat (& other chat improvements)

[pluscubed]

Resolves #98.

Multi-line chat bubbles and input are both working. Tested on iPhone 8 and iPhone XR simulator.

Technical changes made: allow multi-line chat UILabels, change constraints to depend on table cell boundaries, change UITextField to UITextView for multi-line input (with a grey border using a UIView extension configurable in Storyboard), move table/input into a StackView for expandable input and easier keyboard constraint animation

The only new issue is that the placeholder text is gone, due to using UITextView instead of UITextField. There are workarounds, but IMO it looks fine (and it's the default look in Messages anyway).

[pluscubed]

Fixed 2 existing issues:

• Pressing the ">" button dismisses the keyboard and doesn't send the message
• Navigation bar becomes black when pressing back in chat

[pluscubed]

Fixed 1 more existing issue:

• Pressing return on sw keyboard does nothing

[pluscubed]

Fixed a new issue due to multiline UILabels not playing well with insets.

Also set the chat screen's navbar title to the other person's first name.

[kamalca]

Looks good!