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.

test/javascripts/fixtures/user_fixtures.js.es6

@@ -189,6 +189,7 @@ export default {
       card_image_badge: "/images/avatar.png",
       card_image_badge_id: 120,
       muted_usernames: [],
+      ignored_usernames: [],
       invited_by: {
         id: 1,
         username: "sam",