Add Twitter API for the pinned statuses

```
# Only return statuses that have been pinned
GET /api/statuses/user_timeline.json?pinned=true

# Pin
POST /api/statuses/pin/:id

# Unpin
POST /api/statuses/unpin/:id
```

lib/pleroma/web/router.ex

@@ -355,6 +355,9 @@ defmodule Pleroma.Web.Router do
     post("/statuses/unretweet/:id", TwitterAPI.Controller, :unretweet)
     post("/statuses/destroy/:id", TwitterAPI.Controller, :delete_post)
 
+    post("/statuses/pin/:id", TwitterAPI.Controller, :pin)
+    post("/statuses/unpin/:id", TwitterAPI.Controller, :unpin)
+
     get("/pleroma/friend_requests", TwitterAPI.Controller, :friend_requests)
     post("/pleroma/friendships/approve", TwitterAPI.Controller, :approve_friend_request)
     post("/pleroma/friendships/deny", TwitterAPI.Controller, :deny_friend_request)

lib/pleroma/web/twitter_api/representers/activity_representer.ex

@@ -153,6 +153,7 @@ defmodule Pleroma.Web.TwitterAPI.Representers.ActivityRepresenter do
     announcement_count = object["announcement_count"] || 0
     favorited = opts[:for] && opts[:for].ap_id in (object["likes"] || [])
     repeated = opts[:for] && opts[:for].ap_id in (object["announcements"] || [])
+    pinned = activity.id in user.info.pinned_activities
 
     mentions = opts[:mentioned] || []
 
@@ -202,6 +203,7 @@ defmodule Pleroma.Web.TwitterAPI.Representers.ActivityRepresenter do
       "repeat_num" => announcement_count,
       "favorited" => to_boolean(favorited),
       "repeated" => to_boolean(repeated),
+      "pinned" => pinned,
       "external_url" => object["external_url"] || object["id"],
       "tags" => tags,
       "activity_type" => "post",

lib/pleroma/web/twitter_api/twitter_api.ex

@@ -82,6 +82,14 @@ defmodule Pleroma.Web.TwitterAPI.TwitterAPI do
     end
   end
 
+  def pin(%User{} = user, ap_id_or_id) do
+    CommonAPI.pin(ap_id_or_id, user)
+  end
+
+  def unpin(%User{} = user, ap_id_or_id) do
+    CommonAPI.unpin(ap_id_or_id, user)
+  end
+
   def fav(%User{} = user, ap_id_or_id) do
     with {:ok, _fav, %{data: %{"id" => id}}} <- CommonAPI.favorite(ap_id_or_id, user),
          %Activity{} = activity <- Activity.get_create_activity_by_object_ap_id(id) do

lib/pleroma/web/twitter_api/twitter_api_controller.ex

@@ -375,6 +375,30 @@ defmodule Pleroma.Web.TwitterAPI.Controller do
     end
   end
 
+  def pin(%{assigns: %{user: user}} = conn, %{"id" => id}) do
+    with {_, {:ok, id}} <- {:param_cast, Ecto.Type.cast(:integer, id)},
+         {:ok, activity} <- TwitterAPI.pin(user, id) do
+      conn
+      |> put_view(ActivityView)
+      |> render("activity.json", %{activity: activity, for: user})
+    else
+      {:error, message} -> bad_request_reply(conn, message)
+      err -> err
+    end
+  end
+
+  def unpin(%{assigns: %{user: user}} = conn, %{"id" => id}) do
+    with {_, {:ok, id}} <- {:param_cast, Ecto.Type.cast(:integer, id)},
+         {:ok, activity} <- TwitterAPI.unpin(user, id) do
+      conn
+      |> put_view(ActivityView)
+      |> render("activity.json", %{activity: activity, for: user})
+    else
+      {:error, message} -> bad_request_reply(conn, message)
+      err -> err
+    end
+  end
+
   def register(conn, params) do
     with {:ok, user} <- TwitterAPI.register_user(params) do
       conn

lib/pleroma/web/twitter_api/views/activity_view.ex

@@ -243,6 +243,7 @@ defmodule Pleroma.Web.TwitterAPI.ActivityView do
     announcement_count = object["announcement_count"] || 0
     favorited = opts[:for] && opts[:for].ap_id in (object["likes"] || [])
     repeated = opts[:for] && opts[:for].ap_id in (object["announcements"] || [])
+    pinned = activity.id in user.info.pinned_activities
 
     attentions =
       activity.recipients
@@ -300,6 +301,7 @@ defmodule Pleroma.Web.TwitterAPI.ActivityView do
       "repeat_num" => announcement_count,
       "favorited" => !!favorited,
       "repeated" => !!repeated,
+      "pinned" => pinned,
       "external_url" => object["external_url"] || object["id"],
       "tags" => tags,
       "activity_type" => "post",