[chore] The Big Middleware and API Refactor (tm) (#1250)

* interim commit: start refactoring middlewares into package under router

* another interim commit, this is becoming a big job

* another fucking massive interim commit

* refactor bookmarks to new style

* ambassador, wiz zeze commits you are spoiling uz

* she compiles, we're getting there

* we're just normal men; we're just innocent men

* apiutil

* whoopsie

* i'm glad noone reads commit msgs haha :blob_sweat:

* use that weirdo go-bytesize library for maxMultipartMemory

* fix media module paths

CONTRIBUTING.md

@@ -251,7 +251,7 @@ Most of the crucial business logic of the application is found inside the variou
 
 `internal/ap` - ActivityPub utility functions and interfaces.
 
-`internal/api` - Models, routers, and utilities for the client and federated (s2s) APIs. This is where routes are attached to the router, and where you want to be if you're adding a route.
+`internal/api` - Models, routers, and utilities for the client and federated (ActivityPub) APIs. This is where routes are attached to the router, and where you want to be if you're adding a route.
 
 `internal/concurrency` - Worker models used by the processor and other queues.
 
@@ -283,6 +283,8 @@ Most of the crucial business logic of the application is found inside the variou
 
 `internal/messages` - Models for wrapping worker messages.
 
+`internal/middleware` - Gin Gonic router middlewares: http signature checking, cache control, token checks, etc.
+
 `internal/netutil` - HTTP / net request validation code.
 
 `internal/oauth` - Wrapper code/interfaces for OAuth server implementation.