[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](/uploads/70a44ec3abd0e0707cf69f53ee832542/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](/uploads/489b9bb197a092f715bc2160acc7fd42/grab-scroll.webp)