Fix up invalid MySQL migration

Rename files, change import and namespace.

src/Ombi.Store/Migrations/ExternalMySql/20201212163118_Jellyfin.Designer.cs

@@ -4,11 +4,11 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Migrations;
 using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
-using Ombi.Store.Context.Sqlite;
+using Ombi.Store.Context.MySql;
 
-namespace Ombi.Store.Migrations.ExternalSqlite
+namespace Ombi.Store.Migrations.ExternalMySql
 {
-    [DbContext(typeof(ExternalSqliteContext))]
+    [DbContext(typeof(ExternalMySqlContext))]
     [Migration("20201212014227_Jellyfin")]
     partial class Jellyfin
     {

src/Ombi.Store/Migrations/ExternalMySql/20201212163118_Jellyfin.cs

@@ -1,8 +1,8 @@
-using Microsoft.EntityFrameworkCore.Migrations;
-using System;
-using System.Collections.Generic;
+using System;
+using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Migrations;
 
-namespace Ombi.Store.Migrations.ExternalSqlite
+namespace Ombi.Store.Migrations.ExternalMySql
 {
     public partial class Jellyfin : Migration
     {