feat(attribute-completeness): add functionality to use multiple attributes and combine them to one filter

[sfendrich]

Currently the feature attribute-completeness supports a single attribute. It should be extended to support multiple attributes with conjunction semantics.

• [ ] Service-layer already supports multiple attributes, HTTP-layer still needs to be extended.

Please consider also the following:

• [ ] Maybe make attribute name more unique: in dashboard attribute in the URL is not clear that it relates to Attributes Completeness (E.g. attribute-completeness-attribute=myattribute or add it to indicators=attribute-completeness[myattribute]
• [ ] Should attributes have also a project key (similar to topics?)