make site setting for read time word count per minute

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

@@ -228,8 +228,7 @@ const Topic = RestModel.extend({
     const wordCount = this.get('word_count');
     if (!wordCount) return;
 
-    // Avg for 500 words per minute when you account for skimming
-    return Math.floor(wordCount / 500.0);
+    return Math.floor(wordCount / Discourse.SiteSettings.read_time_word_count);
   }.property('word_count'),
 
   toggleBookmark() {

config/locales/server.en.yml

@@ -1169,6 +1169,8 @@ en:
 
     automatically_unpin_topics: "Automatically unpin topics when the user reaches the bottom."
 
+    read_time_word_count: "Word count per minute for calculating estimated reading time."
+
     full_name_required: "Full name is a required field of a user's profile."
     enable_names: "Show the user's full name on their profile, user card, and emails. Disable to hide full name everywhere."
     display_name_on_posts: "Show a user's full name on their posts in addition to their @username."

config/site_settings.yml

@@ -1029,6 +1029,10 @@ uncategorized:
     default: true
     client: true
 
+  read_time_word_count:
+    default: 500
+    client: true
+
 user_preferences:
   default_email_digest_frequency:
     enum: 'DigestEmailSiteSetting'

lib/topic_view.rb

@@ -161,7 +161,7 @@ class TopicView
 
   def read_time
     return nil if @post_number.present? && @post_number.to_i != 1 # only show for topic URLs
-    (@topic.word_count/500).floor if @topic.word_count
+    (@topic.word_count/SiteSetting.read_time_word_count).floor if @topic.word_count
   end
 
   def like_count