Table data url links

[will-moore]

This PR provides 3 steps for adding urls to Table Data.
All parts of this are speculative and up for discussion.

• Add a 'url' description to the column name in your CSV file. Currently I'm using a double %% as the delimiter, but this could be changed? E.g.
  Gene_Symbol %% url=http://www.ensembl.org/id/%s
  This will be parsed by the populate_metadata script, which will attempt to add the data as json in the Column description. E.g.
  Name: Gene_Symbol
  Description: '{"url": "http://www.ensembl.org/id/%s"}'
• Webgateway now adds the description to the json Table data, and attempts to parse the description as json. E.g.

data: {
    rows: [
        [
        "ENSG00000117399",
        "CDC20",
        "44",
        ]
    ],
    descriptions: [
        "",
        {
        url: "http://www.ensembl.org/id/%s"
        },
        ""
    ],
    columns: [
        "Entity_Identifier",
        "Gene_Symbol ",
        "Entity_Number",
    ]
}

• Finally, if a description url is found, webclient uses it to create a link when displaying the gene name.

[joshmoore]

@will-moore : can you rebase this against 29011cc (from #3875) as opposed to those from #3863 ?

[will-moore]

Rebased to #3889.