application/x-www-form-urlencoded support

[owen-d]

What

Adds POST + application/x-www-form-urlencoded support for the querier. This is mainly to maintain parity w/ the upstream prometheus implementation.

This is primarily for

• POST /api/v1/query_range
• POST /api/v1/query

but also will be useful for

• POST /api/v1/series

Closes #1379

example

$  curl -XPOST -s "http://localhost:3100/loki/api/v1/query" --data-urlencode 'query=sum(rate({job="default/promtail"}[10m])) by (level)' | jq
{
  "status": "success",
  "data": {
    "resultType": "vector",
    "result": [
      {
        "metric": {},
        "value": [
          1575661320.349,
          "0.055"
        ]
      }
    ]
  }
}