Fee scheme versioning

[jsugarman]

What

Add the ability to explicitly select/query a fee scheme based on type and version

Why

A nice to have: currently a type and case_date is used to select
the scheme applicable. Where case_date is the earliest rep'
order date attached to a claim.

Although the version information for a fee-calc scheme is currently
available in the fee scheme description, it is not queryable. It
might prove useful to be able to select/query a fee scheme based
purely on type and version. Thinking specifically of implementations
of consuming apps that might ask the user what scheme they are
making a claim under.

It might, therefore, prove useful to:

a. show what version is returned in the JSON

# example JSON response  from fee-scheme endpoint
{
  "count": 1,
  "next": null,
  "previous": null,
  "results": [
    {
      "id": 4,
      "type": "LGFS",
      "version": "10",
      "start_date": "2017-12-01",
      "end_date": null,
      "description": "LGFS Fee Scheme 2017-12"
    }
  ]
}

b. query a specific version using https://laa-fee-calculator<blah>/api/v1/fee-schemes/?type=LGFS&version=10

see version numbering in LGFS regulations which go a bit funky. Could ask the BA for AGFS equivalent info.