Fetch dct:accessRights from proper locations

[streino]

Access rights can be a property of Resource (super-class of Dataset) or Distribution.

Some conditions apply :

Information about licenses and rights SHOULD be provided on the level of Distribution.
Information about licenses and rights MAY be provided for a Dataset in addition to but
not instead of the information provided for the Distributions of that Dataset. Providing
license or rights information for a Dataset that is different from information provided
for a Distribution of that Dataset SHOULD be avoided as this can create legal conflicts.

Also, in the case of DCAT resources coming from the SEMICeu transformation, if I'm not mistaken, the rights are taken at the dataset level from the ISO input and copied at the distribution level in the DCAT output. So we should be able to assume that what we find at the distribution level is in fact applying to the entire record.

Since udata doesn't display distribution-level rights, we'll have to approximate at the dataset-level. It's probably better to be overly careful than miss some restrictions. So we could go with :

• Collect all access rights found at Resource and Distribution levels.
• Uniqify the collection.
• Assign/display the set of access rights at the dataset level.

The other approach would be to store all information in the extras as-is and retrieve what we're interested in displaying when needed.

Finally, the same sort of logic should apply to license, but that part needs to be articulated with existing license logic in udata. So probably another PR.