Preferred IRIs for Xref properties

@egonw and I were discussing preferred IRIs for Xref properties in BridgeDb, and we'd welcome input from other community members on this.

This webservice endpoint http://webservice.bridgedb.org/Human/attributes/En/ENSG00000139618 returns:
```
Description BRCA2, DNA repair associated [Source:HGNC Symbol;Acc:HGNC:1101]
Symbol BRCA2
Type protein_coding
Chromosome 13
```

To describe that call, do these IRIs appear correct?
`description`: http://purl.org/dc/terms/description
`type`: http://www.w3.org/1999/02/22-rdf-syntax-ns#type
`chromosome`: http://purl.obolibrary.org/obo/GO_0005694
`identifier`: http://purl.obolibrary.org/obo/IAO_0000577
`symbol`: http://semanticscience.org/resource/SIO_000105

Does `Symbol` refer to `name`, as in one of these IRIs?
https://vocabularies.wikipathways.org/gpml#textlabel
http://schema.org/name
http://www.biopax.org/release/biopax-level3.owl#name
http://www.biopax.org/release/biopax-level3.owl#displayName
http://www.biopax.org/release/biopax-level3.owl#standardName

Or is it specific to a gene symbol, as in http://edamontology.org/data_1026 from the EDAM ontology?

Note that http://webservice.bridgedb.org/Human/attributes/Ce/CHEBI%3A29108 returns:
```
Symbol calcium
Symbol Ca(2+)
Symbol calcium(2+)
Symbol calcium, doubly charged positive ion
Symbol Ca
Symbol Calcium element
```

and http://webservice.bridgedb.org/Human/attributeSearch/calcium?attrName=Symbol returns:
```
C9J224 Uniprot-TrEMBL Voltage-dependent L-type calcium channel subunit beta-4 (Fragment)
HMDB00464 HMDB Calcium
Q9P0X4 Uniprot-TrEMBL Voltage-dependent T-type calcium channel subunit alpha-1I
CHEBI:4496 ChEBI Dibasic calcium phosphate dihydrate
CHEBI:9679 ChEBI Tricalcium phosphate
Q86V35 Uniprot-TrEMBL Calcium-binding protein 7
O00305 Uniprot-TrEMBL Voltage-dependent L-type calcium channel subunit beta-4
CHEBI:3309 ChEBI Calcium Gluconate
CHEBI:5010 ChEBI Fenprofen calcium
B0QYI5 Uniprot-TrEMBL EF-hand calcium-binding domain-containing protein 6
P57103 Uniprot-TrEMBL Sodium/calcium exchanger 3
HMDB38053 HMDB Calcium trimetaphosphate
CHEBI:3311 ChEBI Calcium carbonate
E7DBM8 Uniprot-TrEMBL Voltage-dependent L-type calcium channel subunit beta-4
CHEBI:4757 ChEBI Edetate calcium disodium
E7EN11 Uniprot-TrEMBL Voltage-dependent L-type calcium channel subunit beta-4
HMDB14471 HMDB Calcium Gluceptate
Q5THR3 Uniprot-TrEMBL EF-hand calcium-binding domain-containing protein 6
F2Z391 Uniprot-TrEMBL Sodium/calcium exchanger 3
```