[#904] Added project hierarchy

- Added new model; RelatedProject. Makes it possible to store a relation between 2 projects, based on IATI related activities (parent, child, sibling, co-funded, third party)
- Added a text on the project main page describing the relations of the project (if any)
- Added a new project-hierarchy page, using regular div's to show the projects and jsPlumb library to visualise the links between the projects.
  - The parent projects are shown on the left
  - The sibling projects and the project itself are shown in the middle
  - The child projects are shown on the right
  - One issue; when the screen becomes too small, the arrows are misplaced (they are static). For now, a message is shown when the screen is too small.
Merge pull request #944 from akvo/904_project_hierarchy

[#904] Added project hierarchy