Add elasticsearch-rails rake tasks

This will allow importing data to ES indexes or recreating them.

We need these tasks to solve
http://community.coopdevs.org/t/busquedas-ofertas-demandas-sin-tener-en-cuenta-acentos/28/6.

Basically, the required mapping were already present in the models long
ago but the indexes were never recreated. While the `Post` model contains

```ruby
settings(
  analysis: {
    analyzer: { normal: {
        tokenizer: "standard",
        # lowercase, unaccent
        filter: %w[lowercase asciifolding]
      }
    }
  }
) do
  mapping do
    indexes :title, analyzer: "normal"
    indexes :description, analyzer: "normal"
    indexes :tags
    indexes :organization_id, type: :integer
  end
```

The offers index does not have the same settings:

```shell
{
  "offers": {
    "aliases": {},
    "mappings": {
      "offer": {
        "properties": {
          "description": {
            "type": "string"
          },
          "organization_id": {
            "type": "long"
          },
          "tags": {
            "type": "string"
          },
          "title": {
            "type": "string"
          }
        }
      }
    },
    "settings": {
      "index": {
        "creation_date": "1484923146831",
        "uuid": "-3bRUbzeRxC5MRcVKwWsjQ",
        "number_of_replicas": "1",
        "number_of_shards": "5",
        "version": {
          "created": "2020099"
        }
      }
    },
    "warmers": {}
  }
}
```

After running:

```shell
$ bundle exec rake environment elasticsearch:import:model CLASS='Offer' FORCE=y
```

The index reflects the new analyzer and mappings:

```shell
{
  "offers": {
    "aliases": {},
    "mappings": {
      "offer": {
        "properties": {
          "description": {
            "type": "string",
            "analyzer": "normal"
          },
          "organization_id": {
            "type": "integer"
          },
          "tags": {
            "type": "string"
          },
          "title": {
            "type": "string",
            "analyzer": "normal"
          }
        }
      }
    },
    "settings": {
      "index": {
        "creation_date": "1501241709326",
        "uuid": "ypU9Nws-SMu8KJ5MdUs8Aw",
        "analysis": {
          "analyzer": {
            "normal": {
              "filter": [
                "lowercase",
                "asciifolding"
              ],
              "tokenizer": "standard"
            }
          }
        },
        "number_of_replicas": "1",
        "number_of_shards": "5",
        "version": {
          "created": "2020099"
        }
      }
    },
    "warmers": {}
  }
}
```

Author: sauloperez

lib/tasks/elasticsearch.rake

@@ -0,0 +1 @@
+require 'elasticsearch/rails/tasks/import'