Add API to manage projects

[zepich]

An API to manage projects is required to create a large number of them (as described in #224 and #226).

The API should have at least the following functions:

• Add a project with name, description, hosts, spam rating factor, and project memberships
• Manage a project by adjusting the name, description, hosts, spam rating factor, and project memberships
• Delete a project

It's possible to enhance the API and allow the management of security and design-related settings, but this would not be the required functionality.

How exactly the API should work is to be defined. It would be possible to allow the creation of a project and import all the project settings using the import functionality (JSON structure of an exported project) via the API. The memberships would have to be managed with additional API functions.