Fixes#784
From now, searching for tags with an empty value will return only not tagged links,
with the search bar showing `x results [not tagged]`.
Note that using the api, the searchtags request parameter must be set to `false` to get the same result.
- [ ] Update API doc
* REST API routes are handle by Slim.
* Every API controller go through ApiMiddleware which handles security.
* First service implemented `/info`, for tests purpose.