Move symmetric encryption/decryption code from leap.common to leap.soledad.
Remove strict dependency on leap.common.

* Encapsulate leap_assert and leap_assert_type so Soledad works without them.
* Remove dependency on leap.common.files.mkdir_p().
* Encapsulate signaling.
* Add changes file.
Update dependencies.
Merge remote-tracking branch 'drebs/feature/2602-remove-strict-dependency-on-leap.common-2' into develop