Better define Entity Page URLs so they can be queried in the UI

[u8sand]

If we want DCC-contributed entity pages to show up without having to manually incorporate all of them we need to define a more strict syntax for the entity page urls.

I'm thinking something along the lines of:

• https://mygenepage.com/${gene.ensembl}
• https://mydrugpage.com/?drug=${drug.label}

Where the available records (gene.ensembl | drug.label) are fully specified in the docs.

We'd support, at the very least, C2M2-defined identifier schemes for the supported entities.

Besides the URL, we'd probably also want a title, description, icon for displaying the specific landing page, e.g.