Merge graylog2-plugin-interfaces into graylog2-server repository

The `graylog-plugin-interfaces` and `graylog2-server` repositories are currently kind of artificially seperated which leads to significant overhead during development.

This pull request merges `graylog2-plugin-interfaces` into `graylog2-server` as a Maven submodule and moves the server related code into another submodule while retaining the history of both repositories.

The result is a Maven multi-module  project containing a parent POM (in the repository's root) and two submodules (`plugin-interfaces` and `server`). With the new module structure, dependency information (e. g. versions of certain dependencies or Maven plugins) can be shared across the submodules. The new layout also simplifies keeping the versions in sync and synchronizing the plugin-interface with the actual provider/server code.