Added REST API for tasks

There are several major changes in this commit:
* db api refactored to extract common code to common methods
* added REST API layer for tasks
* creation of story now triggers creation of default associated task
* it is possible to filter stories by project.id
* it is possible to filter tasks by story.id
* all new features have functional test coverage
* updated rest api docs to include only functioning endpoints

Change-Id: Ice2ebfa174e35944bebd71e3cf4b809a3e825761
Merge "Added REST API for tasks"