ClinVar & diseases support enhancements

[krassowski]

Following recent refactoring of ClinVar import and DB structure, if time permits:

• [x] display gold starts in mutation metadata
• [x] allow filtering by the gold stars number
• [x] maybe add new links to ClinVar pages (not only to dbSNP)
• [ ] display disease cross-references (external identifiers and links)
• [ ] allow searching for diseases by disease external identifier
• [ ] import and test the RSV/VSV identifiers
• [ ] display variant "overall" significance (aggregated from variant-disease associations on VCV level)