$form_state is now a classed object

HI. 

A lot of interfaces now explicitly typehint $form_state with FormStateInterface  due this  issue https://www.drupal.org/node/2225353 and more info in this change record: https://www.drupal.org/node/2310411 

I just update the  $form_state  where the interfaces  typehint them.

There still are some function without this change: 

```bash
grep -Hirn "function.*form_state" . | grep -v FormStateInterface
./flag.module:260:function flag_form_user_admin_permissions_alter(&$form, &$form_state, $form_id) {
./flag.module:326:function flag_form_node_type_form_alter(&$form, &$form_state, $form_id) {
./flag.module:377:function flag_field_attach_form($entity_type, $entity, &$form, &$form_state, $langcode) {
./flag.module:482:function flag_field_attach_submit($entity_type, $entity, $form, &$form_state) {
./src/Form/FlagFormBase.php:230:  public function updateSelectedPluginType($form, $form_state) {
./src/Form/FlagFormBase.php:237:  public function submitSelectPlugin(array $form, array &$form_state) {
./src/Plugin/views/field/FlagViewsFlaggedField.php:38:  public function buildOptionsForm(&$form, &$form_state) {
./src/Plugin/views/field/FlagViewsLinkField.php:49:  public function buildOptionsForm(&$form, &$form_state) {
./src/Plugin/views/filter/FlagViewsFilter.php:27:  public function buildOptionsForm(&$form, &$form_state) {
./src/Plugin/views/relationship/FlagViewsRelationship.php:26:  public function buildOptionsForm(&$form, &$form_state) {
```
I've been testing the module and all seems to work,  i can change the rest of $form_state's if you want. 

Before to test this i would recommend to merge the andypost PR first: 
https://github.com/socketwench/flag-drupal8/pull/50