FEATURE: Add ignored user list to the User's preference page (#7107)

* FEATURE: Add ignored user list to the User's preference page

## Why?

Part of: https://meta.discourse.org/t/ability-to-ignore-a-user/110254

We want to add list of Ignored users under or along with the muted users preferences section.
This way Users can find and update their list of ignored users.

## UI


![gif](https://user-images.githubusercontent.com/45508821/53746179-8e9b3c00-3e98-11e9-9e90-94b8520896a6.gif)

## Open questions

Two of many options to represent a list of ignored users is that we can:

1. We can represent the ignored user list as a table with the ability to `un-ignore` but NOT to add new ignored users.
2. We can keep it functioning as the `muted user list` where you can `un-ignore` or `ignore` users.

config/locales/client.en.yml

@@ -722,6 +722,8 @@ en:
       users: "Users"
       muted_users: "Muted"
       muted_users_instructions: "Suppress all notifications from these users."
+      ignored_users: "Ignored"
+      ignored_users_instructions: "Lists all your ignored users."
       muted_topics_link: "Show muted topics"
       watched_topics_link: "Show watched topics"
       tracked_topics_link: "Show tracked topics"
@@ -761,6 +763,7 @@ en:
         emails: "Emails"
         notifications: "Notifications"
         categories: "Categories"
+        users: "Users"
         tags: "Tags"
         interface: "Interface"
         apps: "Apps"

config/routes.rb

@@ -403,6 +403,7 @@ Discourse::Application.routes.draw do
     get "#{root_path}/:username/preferences/emails" => "users#preferences", constraints: { username: RouteFormat.username }
     get "#{root_path}/:username/preferences/notifications" => "users#preferences", constraints: { username: RouteFormat.username }
     get "#{root_path}/:username/preferences/categories" => "users#preferences", constraints: { username: RouteFormat.username }
+    get "#{root_path}/:username/preferences/users" => "users#preferences", constraints: { username: RouteFormat.username }
     get "#{root_path}/:username/preferences/tags" => "users#preferences", constraints: { username: RouteFormat.username }
     get "#{root_path}/:username/preferences/interface" => "users#preferences", constraints: { username: RouteFormat.username }
     get "#{root_path}/:username/preferences/apps" => "users#preferences", constraints: { username: RouteFormat.username }

config/site_settings.yml

@@ -532,6 +532,7 @@ users:
   ignore_user_enabled:
     hidden: true
     default: false
+    client: true
 
 groups:
   enable_group_directory: