[Frontend / Backend] Billing dashboard

[LuckyShuii]

When you click on the user image at the top right on a page, if you're logged in, there is a "billing" tab.
The goal here is to code it! yay

This new page should contain the payment history of the user for his plan, see his current plan and the possibility of changing it for an other one (either redirect on the plan page for a first version, or the possibility to do it directly on this page would be noice)

The user has to be able to cancel his subscription anytime, but he'll keep his access until the end of the month or smth