fix(6.x forms) invalid dependencies with maven 3.3 (#149)

Subscription forms jars built with maven 3.3.x have invalid maven
dependencies (community jars used for the shade are also part of the
transitive dependencies)

- use <optional>true<optional> to avoid that
- add missing dependencies to SP forms-server artifact

Covers [BS-16017](https://bonitasoft.atlassian.net/browse/BS-16017)