Add the `UserStatus` model

This model will hold the status of a user, including these fields:

- emoji: always present, with a default value
- user: always present, foreign key to user
- message: optional, maximum length of 100

The table also stores

- cached_markdown_version
- message_html

For rendering markdown in the `message` field.

app/models/user_status.rb

+# frozen_string_literal: true
+
+class UserStatus < ActiveRecord::Base
+  include CacheMarkdownField
+
+  belongs_to :user
+
+  validates :user, presence: true
+  validates :emoji, inclusion: { in: Gitlab::Emoji.emojis_names }
+  validates :message, length: { maximum: 100 }, allow_blank: true
+
+  cache_markdown_field :message, pipeline: :single_line
+end

db/migrate/20180713092803_create_user_statuses.rb

+# frozen_string_literal: true
+
+class CreateUserStatuses < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    create_table :user_statuses, id: false, primary_key: :user_id do |t|
+      t.references :user,
+                   foreign_key: { on_delete: :cascade },
+                   null: false,
+                   primary_key: true
+      t.integer :cached_markdown_version, limit: 4
+      t.string :emoji, null: false, default: 'speech_balloon'
+      t.string :message, limit: 100
+      t.string :message_html
+    end
+  end
+end

db/schema.rb

@@ -2048,6 +2048,13 @@ ActiveRecord::Schema.define(version: 20180722103201) do
   add_index "user_interacted_projects", ["project_id", "user_id"], name: "index_user_interacted_projects_on_project_id_and_user_id", unique: true, using: :btree
   add_index "user_interacted_projects", ["user_id"], name: "index_user_interacted_projects_on_user_id", using: :btree
 
+  create_table "user_statuses", primary_key: "user_id", force: :cascade do |t|
+    t.integer "cached_markdown_version"
+    t.string "emoji", default: "speech_balloon", null: false
+    t.string "message", limit: 100
+    t.string "message_html"
+  end
+
   create_table "user_synced_attributes_metadata", force: :cascade do |t|
     t.boolean "name_synced", default: false
     t.boolean "email_synced", default: false
@@ -2349,6 +2356,7 @@ ActiveRecord::Schema.define(version: 20180722103201) do
   add_foreign_key "user_custom_attributes", "users", on_delete: :cascade
   add_foreign_key "user_interacted_projects", "projects", name: "fk_722ceba4f7", on_delete: :cascade
   add_foreign_key "user_interacted_projects", "users", name: "fk_0894651f08", on_delete: :cascade
+  add_foreign_key "user_statuses", "users", on_delete: :cascade
   add_foreign_key "user_synced_attributes_metadata", "users", on_delete: :cascade
   add_foreign_key "users", "application_setting_terms", column: "accepted_term_id", name: "fk_789cd90b35", on_delete: :cascade
   add_foreign_key "users_star_projects", "projects", name: "fk_22cd27ddfc", on_delete: :cascade