mu4e: fix `q' binding in About buffer

mu4e's About buffer uses org-mode and used to locally bind `q' to bury
the buffer.  However, altering local key map via local-set-key
shares binding with all buffers using the same major mode.  As a result,
after executing About mu4e once, pressing `q' in any regular org-mode
buffer was also burying it instead of inserting the letter `q'.

This commit fixes the problem by creating a new mu4e-about-mode
derived from org-mode and by defining `q' only for that mode.