Change default help bind to ?

Which reminds me, search and search-regex need help.

C-c doesn't differentiate between user and automatically set key binds
so, if you've already got a config do ":bind ? help" to see it.
Downgrade new tag notification to .debug

Two reasons

- addtag holds config_lock, log.info will take sync_lock to open a
  window, which is deadlock prone (this could be solved easily by moving
the log.info out of that area)

- new tags are only created in response to user changes (i.e. adding
  feeds, tagging items with a new tag) so they know it's been added, but
this message doesn't actually correlate with items showing up in the
interface
Convert deltag warning to .debug

Same reason as previous commit, deadlocks, but this should also never
happen anymore without user action (deltag only comes on removal of a
feed).
Fix search prompts

- Can now suppress completions in input_callback (wouldn't make sense to
  try and complete search terms)

- Search bind '/' properly brings up prompt. The :search and
  :search-regex commands already worked, they just didn't work with a
keybind (since there was no way to actually give it a search term
without the prompt).

- Bring back 'm' to manually toggle marking items