Implement APIv4 infrastructure (#5191)

* Implement APIv4 infrastructure

* Update parameter requirement functions per feedback

api4/params.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2017 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api4
+
+import (
+	"net/http"
+
+	"github.com/gorilla/mux"
+)
+
+type ApiParams struct {
+	UserId    string
+	TeamId    string
+	ChannelId string
+	PostId    string
+	FileId    string
+	CommandId string
+	HookId    string
+	EmojiId   string
+}
+
+func ApiParamsFromRequest(r *http.Request) *ApiParams {
+	params := &ApiParams{}
+
+	props := mux.Vars(r)
+
+	if val, ok := props["user_id"]; ok {
+		params.UserId = val
+	}
+
+	if val, ok := props["team_id"]; ok {
+		params.TeamId = val
+	}
+
+	if val, ok := props["channel_id"]; ok {
+		params.ChannelId = val
+	}
+
+	if val, ok := props["post_id"]; ok {
+		params.PostId = val
+	}
+
+	if val, ok := props["file_id"]; ok {
+		params.FileId = val
+	}
+
+	if val, ok := props["command_id"]; ok {
+		params.CommandId = val
+	}
+
+	if val, ok := props["hook_id"]; ok {
+		params.HookId = val
+	}
+
+	if val, ok := props["emoji_id"]; ok {
+		params.EmojiId = val
+	}
+
+	return params
+}