[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

internal/middleware/useragent.go

@@ -0,0 +1,39 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package middleware
+
+import (
+	"errors"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// UserAgent returns a gin middleware which aborts requests with
+// empty user agent strings, returning code 418 - I'm a teapot.
+func UserAgent() gin.HandlerFunc {
+	// todo: make this configurable
+	return func(c *gin.Context) {
+		if ua := c.Request.UserAgent(); ua == "" {
+			code := http.StatusTeapot
+			err := errors.New(http.StatusText(code) + ": no user-agent sent with request")
+			c.AbortWithStatusJSON(code, gin.H{"error": err.Error()})
+		}
+	}
+}