Add default location config, fix dropped meetings

- Adds ability to configure per-email default locations, plus a global
default location in new "defaultlocations.json" file.
- Fixes issue where meetings would be dropped if no location specified.
- Unit tests updated, and additional tests are created to detect
regressions in "dropped" meeting fixes thus far.
Merge branch 'master' of https://github.com/patrickmeiring/belated