fix OMIM details on gene page

[mathob]

Describe the bug
On a gene page e.g for ACTA2 there are two OMIM-related problems:

i) there is no OMIM link at the top - at seqr.broadinstitute.org there is a link to the left of "PubMed". I know there has been discussion about the feasibility of including OMIM data in this instance, but even if a map of symbol to OMIM gene id is unavailable, we should still be able to use the symbol only to add a useful link of the form:

https://www.omim.org/search?search=ACTA2&field=approved_gene_symbol

(Note that the clinvar link is like this, just a search with the symbol)

ii) the disease associations section says "OMIM No disease associations" whereas I suspect it should say "OMIM data not available" (or perhaps there should not be an OMIM section at all)

Link to page(s) where bug is occurring

Screenshots

see #125