[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/Admin/Views/Providers/View.cshtml

@@ -0,0 +1,13 @@
+@model ProviderViewModel
+@{
+    ViewData["Title"] = "Provider: " + Model.Provider.Name;
+}
+
+<h1>Provider <small>@Model.Provider.Name</small></h1>
+
+<h2>Information</h2>
+@await Html.PartialAsync("_ViewInformation", Model)
+<form asp-action="Delete" asp-route-id="@Model.Provider.Id"
+      onsubmit="return confirm('Are you sure you want to delete this provider (@Model.Provider.Name)?')">
+    <button class="btn btn-danger" type="submit">Delete</button>
+</form>