Give a dynamic count of recipients as selected categories change

This is a great idea! Thanks for the initial patch.

A few comments regarding the implementation:

1. +++ b/src/Form/MassContactForm.php
   @@ -445,6 +455,26 @@ class MassContactForm extends ContentEntityForm {
   +  *   Ajax callback ¶
   

   Nit pick: Trailing space here, and also this should mention what the callback does :)

2. +++ b/src/Form/MassContactForm.php
   @@ -445,6 +455,26 @@ class MassContactForm extends ContentEntityForm {
   +  public function countRecipients(array &$form, FormStateInterface $form_state) {
   

   Since there can be non-role-based 'grouping methods', this function will probably need to figure out which plugins are selected, and then call out to each selected plugin for a complete list of UIDs, and then do a count of unique uids returned by all methods.

   Checkout Drupal\mass_contact\Plugins\MassContact\GroupingMethod\Role. We might need to add a new interface or something that defines the ability of a plugin to provide this ability.

3. +++ b/src/Form/MassContactForm.php
   @@ -445,6 +455,26 @@ class MassContactForm extends ContentEntityForm {
   +        $results = db_query("SELECT COUNT(`roles_target_id`) as `usercount` FROM `user__roles` WHERE `roles_target_id` = '$category'")->fetchAll();
   

   When this logic is moved into the role plugin, it should at the very least use Database::select() instead of a raw SQL query I think.