Cleans up some routing, mvc base classes, gets controller specific model binding working, applies our own application model to our controllers

src/Umbraco.Web.Common/ApplicationModels/UmbracoApiBehaviorApplicationModelProvider.cs

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+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.ApplicationModels;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using System.Collections.Generic;
+using System.Linq;
+using Umbraco.Core;
+using Umbraco.Web.Common.Attributes;
+using Umbraco.Web.Common.ModelBinding;
+
+namespace Umbraco.Web.Common.ApplicationModels
+{
+    public class NewtonsoftJsonModelBinderConvention : IActionModelConvention
+    {
+        public void Apply(ActionModel action)
+        {
+            foreach(var p in action.Parameters)
+            {
+                if (p.BindingInfo?.BindingSource == BindingSource.Body)
+                {
+                    p.BindingInfo.BinderType = typeof(UmbracoJsonModelBinder);
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// A custom application model provider for Umbraco controllers
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Conventions will be applied to controllers attributed with <see cref="UmbracoApiControllerAttribute"/>
+    /// </para>
+    /// <para>
+    /// This is nearly a copy of aspnetcore's ApiBehaviorApplicationModelProvider which supplies a convention for the
+    /// [ApiController] attribute, however that convention is too strict for our purposes so we will have our own.
+    /// </para>
+    /// </remarks>
+    public class UmbracoApiBehaviorApplicationModelProvider : IApplicationModelProvider
+    {
+        public UmbracoApiBehaviorApplicationModelProvider(IModelMetadataProvider modelMetadataProvider)
+        {
+            // see see https://docs.microsoft.com/en-us/aspnet/core/web-api/?view=aspnetcore-3.1#apicontroller-attribute
+            // for what these things actually do
+            // NOTE: we don't have attribute routing requirements and we cannot use ApiVisibilityConvention without attribute routing
+
+            ActionModelConventions = new List<IActionModelConvention>()
+            {
+                new ClientErrorResultFilterConvention(), // TODO: Need to determine exactly how this affects errors
+                new InvalidModelStateFilterConvention(), // automatically 400 responses if ModelState is invalid before hitting the controller
+                new ConsumesConstraintForFormFileParameterConvention(), // If an controller accepts files, it must accept multipart/form-data.
+                new InferParameterBindingInfoConvention(modelMetadataProvider), // no need for [FromBody] everywhere, A complex type parameter is assigned to FromBody
+
+                new NewtonsoftJsonModelBinderConvention()
+            };
+
+            // TODO: Need to determine exactly how this affects errors
+            var defaultErrorType = typeof(ProblemDetails);
+            var defaultErrorTypeAttribute = new ProducesErrorResponseTypeAttribute(defaultErrorType);
+            ActionModelConventions.Add(new ApiConventionApplicationModelConvention(defaultErrorTypeAttribute));
+        }
+
+        /// <summary>
+        /// Will execute after <see cref="DefaultApplicationModelProvider"/>
+        /// </summary>
+        public int Order => 0;
+
+        public List<IActionModelConvention> ActionModelConventions { get; }
+
+        public void OnProvidersExecuted(ApplicationModelProviderContext context)
+        {
+        }
+
+        public void OnProvidersExecuting(ApplicationModelProviderContext context)
+        {
+            foreach (var controller in context.Result.Controllers)
+            {
+                if (!IsUmbracoApiController(controller))
+                    continue;
+
+                foreach (var action in controller.Actions)
+                {
+                    foreach (var convention in ActionModelConventions)
+                    {
+                        convention.Apply(action);
+                    }   
+                }
+                    
+            }
+        }
+
+        private bool IsUmbracoApiController(ControllerModel controller) => controller.Attributes.OfType<UmbracoApiControllerAttribute>().Any();
+    }
+}