[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/Sql/dbo/Stored Procedures/ProviderUser_ReadCountByProviderIdEmail.sql

@@ -0,0 +1,21 @@
+CREATE PROCEDURE [dbo].[ProviderUser_ReadCountByProviderIdEmail]
+    @ProviderId UNIQUEIDENTIFIER,
+    @Email NVARCHAR(256),
+    @OnlyUsers BIT
+AS
+BEGIN
+    SET NOCOUNT ON
+
+    SELECT
+        COUNT(1)
+    FROM
+        [dbo].[ProviderUser] OU
+    LEFT JOIN
+        [dbo].[User] U ON OU.[UserId] = U.[Id]
+    WHERE
+        OU.[ProviderId] = @ProviderId
+        AND (
+            (@OnlyUsers = 0 AND @Email IN (OU.[Email], U.[Email]))
+            OR (@OnlyUsers = 1 AND U.[Email] = @Email)
+        )
+END