chatview: follow new design for invitations and buttons

- Update invitation design.

- Update colors. Note: non-action-buton class is used or the back
  arrow button, but for the new design its color can be shared with
  action-button.

- Remove separation line between messages area and send container.

- Update textarea input for writing messages. TODO: text should be
  vertically centered, but it can be tricky since textarea doesn't
  handle centering and we need to keep responsive height.

- Update buttons: update icons, hide send button when there is nothing
  to send.

Gitlab: #241

Change-Id: I898739961a6b4b69b5bf4006dfcb298639911322