Add EMBL-EBI OLS as explicit reference for cell types/tissues

[kcs3]

We would like to add EMBL-EBI OLS lookup to cell type and tissue subject pages (both are cell-subset)

An example URL for cell ontology CL_0000624 is
https://www.ebi.ac.uk/ols/ontologies/cl/terms?iri=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FCL_0000624

UBERON codes will be redirected to OLS automatically by the existing URL:
That is, http://purl.obolibrary.org/obo/UBERON_0000178 redirects to
https://www.ebi.ac.uk/ols/ontologies/uberon/terms?iri=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FUBERON_0000178
but we don't need to handle it differently, we can still add an explicit link to OLS

[kcs3]

moved issue to signatures repository