Generify some parts of the REST data model

[maxhbr]

to allow rest clients to handle the JSON response (e.g. for projects) in an generic and uniform way it would be good to save fileds which contain similarly structured data in some generic dict / object. This simplefies the client implementation and also decoples the thrift data model from the rest data model.

Examples for that are the user roles, which are also handled in an generic way by the roles-map:

    optional map<string,set<string>> roles

One could use this generic data model to also represent createdBy, projectResponsible, leadArchitect, moderators, contributors, securityResponsibles.

same for:

• time-points: preevaluationDeadline, systemTestStart, systemTestEnd, deliveryStart, phaseOutSince
• links: homepage, wiki

already done for:

• externalIds