Add notifications for follow requests (#1729)

* Add notifications for follow requests
Issue #1719

* Revert item_follow_request layout, create new layout for follow request notifications

* Migrate follow request interaction from notification to observable pattern

* Filter follow request notifications by default

* Add missing cases for system notification generation

* Format code

app/src/main/java/com/keylesspalace/tusky/network/MastodonApi.kt

@@ -383,6 +383,16 @@ interface MastodonApi {
             @Path("id") accountId: String
     ): Call<Relationship>
 
+    @POST("api/v1/follow_requests/{id}/authorize")
+    fun authorizeFollowRequestObservable(
+            @Path("id") accountId: String
+    ): Single<Relationship>
+
+    @POST("api/v1/follow_requests/{id}/reject")
+    fun rejectFollowRequestObservable(
+            @Path("id") accountId: String
+    ): Single<Relationship>
+
     @FormUrlEncoded
     @POST("api/v1/apps")
     fun authenticateApp(