Abstracted cloud storage - Amazon/Google/Azure/Local (#708)

* wip

* add in storage system

* ui fixes

* fix settings ui

* Add Files Crud UI

* add titles

* link files to users

* add migration

* set blob to public

* remove base 64 read code

* fix file query model init

* move view model to own file

* fix local root path

* use datadir for local storage

* move to services

* add direct file url

* try fix tests

* remove magic string

* remove other magic strings

* show error message on unsupported provider

* fix asp net version

* redirect to storage settings if provider is not supported

* start writing tests

* fix tests

* fix test again

* add some more to the tests

* more tests

* try making local provider work on tests

* fix formfile

* fix small issue with returning deleted file

* check if returned data is null for deleted file

* validate azure Container name

* more state fixes

* change azure test trait

* add tmp file url generator

* fix tests

* small clean

* disable amazon and  google


comment out unused code for now


comment out google/amazon

BTCPayServer/Controllers/StorageController.cs

@@ -0,0 +1,24 @@
+using System.Threading.Tasks;
+using BTCPayServer.Storage.Services;
+using Microsoft.AspNetCore.Mvc;
+
+namespace BTCPayServer.Storage
+{
+    [Route("Storage")]
+    public class StorageController
+    {
+        private readonly FileService _FileService;
+
+        public StorageController(FileService fileService)
+        {
+            _FileService = fileService;
+        }
+
+        [HttpGet("{fileId}")]
+        public async Task<IActionResult> GetFile(string fileId)
+        {
+            var url = await _FileService.GetFileUrl(fileId);
+            return new RedirectResult(url);
+        }
+    }
+}