index all `clinvar.rcv.conditions.identifiers` fields, including mondo and mesh

[colleenXu]

The best way to address https://github.com/biothings/biothings_explorer/issues/548 is if BTE can use and query the clinvar.rcv.conditions.identifiers.mondo (example) and clinvar.rcv.conditions.identifiers.mesh fields.

However, these fields don't seem to be indexed:

• they're not listed in the metadata/fields response
• _exists_ queries for those fields don't return any hits (mondo, mesh)
• no hits are returned when trying to query using those fields (mondo, mesh)

So at minimum, it would help to have these two fields indexed.

I'm not sure if there's more non-indexed fields for clinvar.rcv.conditions.identifiers data that I'm not aware of. If there are more, I'd like to kow what they are: they may also be useful to have indexed / used by BTE...

[everaldorodrigo]

The fields mondo and mesh are added to the mapping here: #181