Add support for Shlink v2 together with v1

[acelaya]

• On errors, try to get the error code from the type property, and fall back to the error property if not found.
• On validation errors, try to get the list of invalidElements, and change the UI accordingly. (To be done on next release).
• When the first request is performed on the API client, use v2 on the URL. If it returns a 404 and the content-type is not application/problem+json, transparently fall back to v1 and make any subsequent request to v1 too.
• Make shlink-web-client take into consideration the fact that lat and long might be numbers and not strings. (already working like this)
• Hide "preview" menu item if shlink version coming from health endpoint is 2.