Help text isn't visible in introduction message screen
@Megalox reported this on https://code.briarproject.org/akwizgran/briar/issues/570#note_16887:
there is a help text on the mesage input screen, but it is partly hidden by the input field and I can't scroll the upper part, I have to close the keyboard to read the last line.
Let's replace the help text with a hint in the input field - something like "Add a message (optional)".