[Provider] Server entities and models (#1370)

* Mock out provider models and service

* Implement CreateAsync, CompleteSetupAsync, UpdateAsync, InviteUserAsync and ResendInvitesAsync

* Implement AcceptUserAsync and ConfirmUsersAsync

* Implement SaveUserAsync and DeleteUserAsync

* Add email templates

* Add admin operations for providers

* Fix mail template names

* Rename roles

* Verify provider has provideradmin

* Add self hosted check to admin controller

* Resolve review comments

* Update sql queries

* Change create provider to use email instead of userId

src/Core/Repositories/IProviderRepository.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Bit.Core.Models.Table.Provider;
+
+namespace Bit.Core.Repositories
+{
+    public interface IProviderRepository : IRepository<Provider, Guid>
+    {
+        Task<ICollection<Provider>> SearchAsync(string name, string userEmail, int skip, int take);
+    }
+}