`ctrl-a` and `ctrl-e` should jump to the beginning/end of message instead of line …

When typing more than of line of text in the text-input field, the shortcuts `ctrl-a` and `ctrl-e` should jump to the beginning or respectively end of the typed message instead of the beginning or end of the line the cursor is currently in.

Start by locating the text-input field and its keyboard handling for ctrl-a and ctrl-e, then reproduce the behavior with a message longer than one line. Done means both shortcuts move to the beginning or end of the entire typed message rather than only the current line.

Written by the indexing model from the issue text.