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.