version: bump to 0.18~rc0

Start the promised feature freeze
debian: changelog stanza for 0.18~rc0-1

- make versions match to pacify release-checks.sh
- close a few more bugs.
- fix one HTMLism
debian: NEWS item about database upgrade
NEWS: bump version

As usual, we'll just edit the version in place for release candidates.
python: bump version
doc: remove conf.pyc on clean

This build artifict messes up the packaging process for (at least)
Debian if not removed on clean.
Merge tag '0.18_rc0'

notmuch 0.18~rc0 release
building from git: use --abbrev=7 for version string

Users may have set core.abbrev=n, where n != 7 in their git config
file(s) which would give them different than expected version strings
when building notmuch from git. This fixes the commit hash part of
version string to 7 hexadecimal values.
NEWS: Ido initialization for Emacs 23.[123]
NEWS: replies to encrypted messages encrypted by default
NEWS: folder:, path:, and database upgrade

Related news together.
NEWS: mbox files are no longer supported
NEWS: message header parser changes
NEWS: add overview section for 0.18

This mentions the things I (subjectively) thought were most likely to
either annoy people or make it worth upgrading.
NEWS: notmuch insert respects maildir.synchronize_flags
NEWS: cli exit status codes
NEWS: emacs: push mark before signature on reply
NEWS: message piping working directory
NEWS: insert and new refuse invalid tags
NEWS: make it explicit that wildcard matching is no longer supported

Wildcard matching was a feature of the probabilistic prefix, and we no
longer have it for the boolean prefix. Also note that top-level folder
can now be searched.