Add ClinicalTrials.gov to handled MIRIAMs

Reference of the identifier:

https://www.ebi.ac.uk/miriam/main/datatypes/MIR:00000137

created branch 580-add-clinicaltrials-gov-to-handled-miriams

mentioned in merge request !507 (merged)

closed via merge request !507 (merged)

mentioned in commit 439194e7