FEATURE: allow users to control how many previous replies they get

- always means we always send previous replies with every email
- never means we do not
- "unless previously sent" ... is the default, in which we only email you each reply once

The default_email_previous_replies site setting can control this toggle

app/assets/javascripts/discourse/controllers/preferences.js.es6

@@ -62,6 +62,12 @@ export default Ember.Controller.extend(CanCheckEmails, {
     return this.siteSettings.available_locales.split('|').map(s => ({ name: s, value: s }));
   },
 
+  previousRepliesOptions: [
+    {name: I18n.t('user.email_previous_replies.always'), value: 0},
+    {name: I18n.t('user.email_previous_replies.unless_emailed'), value: 1},
+    {name: I18n.t('user.email_previous_replies.never'), value: 2}
+  ],
+
   digestFrequencies: [{ name: I18n.t('user.email_digests.daily'), value: 1 },
                       { name: I18n.t('user.email_digests.every_three_days'), value: 3 },
                       { name: I18n.t('user.email_digests.weekly'), value: 7 },

app/assets/javascripts/discourse/models/user.js.es6

@@ -159,6 +159,7 @@ const User = RestModel.extend({
             'email_digests',
             'email_direct',
             'email_private_messages',
+            'email_previous_replies',
             'dynamic_favicon',
             'enable_quoting',
             'disable_jump_reply',

app/assets/javascripts/discourse/templates/user/preferences.hbs

@@ -176,6 +176,10 @@
           </div>
         {{/if}}
       {{/if}}
+      <div class='controls controls-dropdown'>
+        <label>{{i18n 'user.email_previous_replies.title'}}</label>
+        {{combo-box valueAttribute="value" content=previousRepliesOptions value=model.user_option.email_previous_replies}}
+      </div>
       {{preference-checkbox labelKey="user.email_private_messages" checked=model.user_option.email_private_messages}}
       {{preference-checkbox labelKey="user.email_direct" checked=model.user_option.email_direct}}
       <span class="pref-mailing-list-mode">{{preference-checkbox labelKey="user.mailing_list_mode" checked=model.user_option.mailing_list_mode}}</span>
@@ -188,6 +192,9 @@
           {{i18n 'user.email.frequency_immediately'}}
         {{/if}}
       </div>
+
+
+
     </div>
 
     <div class="control-group notifications">