[SMS App] Make SMS message field scrollable

Summary

This patch makes SMS message field scrollable and limits its height to ⅓
of window height (or ⅔ for smaller windows) so messages in conversation
are shown even when writing very long messages.

Test Plan (and screenshots)

<small>These screenshots are in APNG and WEBP formats. Download them if
your browser does not support this format.</small>

Before:

Message field height was not limited. Message field could be even higher
than window. That makes first lines of this field and messages in
conversation inaccessible.

!grab-noscroll.apng

After:

Message field height is limited to ⅓ or ⅔ of window height. When text
content reaches this threshold, scroll bars will show up.

!grab-scroll.webp