Introduce consts for filter types

[trzcina]

It would be really useful if we could use enumerated filter types like this:

enum Filter {
  EQUAL = "equals",
  NOT_EQUAL = "notEqual",
  //...
}

and use it like this:

filters: JSON.stringify({
      slug: {
        type: Filter.EQUAL,
        filter: 123
      }
    }

instead of the current way:

filters: JSON.stringify({
      slug: {
        type: 'equals',
        filter: 123,
      }
    }