feat: configure mail fetcher with offlineimap

- Use tags to help on the Qrexec policy notation;
- Create AppVMs also to fetch and send emails, useful for OfflineIMAP
  that requires sync;
- OfflineIMAP is smart enough depending on the server, such as Gmail;
- Quote options managed by the user such as password fields as they
  could contain spaces; and
- Default fetching method to always keep files on the remote to avoid
  users being surprised about the fetcher behavior or losing data.

scripts/qubesbuilder-gen.sh

@@ -28,7 +28,7 @@ fi
 ignored="$(git ls-files --exclude-standard --others --ignored salt/)"
 untracked="$(git ls-files --exclude-standard --others salt/)"
 unwanted="$(printf '%s\n%s\n' "${ignored}" "${untracked}" |
-  grep -E "^salt/\S+/(README.md|.*\.sls|files/.*)$" | cut -d "/" -f2 |
+  grep -E "^salt/\S+/(README.md|version)$" | cut -d "/" -f2 |
   sort -u)"
 group="$(./scripts/spec-get.sh dom0 group)"
 projects="$(find salt/ -mindepth 1 -maxdepth 1 -type d | sort -d |