update payloads for msg spec 5

ref #6926, describing the payload problem in more detail. I don't think we can reasonably fit a complete replacement for payloads in 3.0, so this just does a bit of cleanup and documentation.

- remove auto_rewrite_input
- remove doctest_mode
- remove exit key from ask_exit
- rename 'edit_magic' to 'edit'
- remove screen_lines from page

remaining payloads ('edit', 'ask_exit', 'set_next_input', 'page') are documented as deprecated in the msg spec, pending their proper replacement in the future. Given that, the current implementation will only be deprecated in favor of the (not yet implemented) new way, rather than removed.


closes #1730
doctest mode does three things:

- set plain prompts
- set xmode plain
- disable pretty-printing

setting prompts doesn't make sense when the kernel isn't a frontend, but the other two do. I can raise a UsageError if it seems better to disable doctest mode than allowing just the kernel-specific parts to take place.