Introduced interface on BackOfficeUserManager (#8913)

* Introduced IBackOfficeUserManager

* Fixed test

* Moved class into own file

Co-authored-by: Elitsa Marinovska <elm@umbraco.dk>

src/Umbraco.Web.BackOffice/Controllers/UsersController.cs

@@ -72,7 +72,7 @@ namespace Umbraco.Web.BackOffice.Controllers
         private readonly IMediaService _mediaService;
         private readonly IContentService _contentService;
         private readonly GlobalSettings _globalSettings;
-        private readonly BackOfficeUserManager _backOfficeUserManager;
+        private readonly IBackOfficeUserManager _backOfficeUserManager;
         private readonly ILogger _logger;
         private readonly LinkGenerator _linkGenerator;
 
@@ -95,7 +95,7 @@ namespace Umbraco.Web.BackOffice.Controllers
             IMediaService mediaService,
             IContentService contentService,
             IOptions<GlobalSettings> globalSettings,
-            BackOfficeUserManager backOfficeUserManager,
+            IBackOfficeUserManager backOfficeUserManager,
             ILogger logger,
             LinkGenerator linkGenerator)
         {