Gorilla Web Toolkit was archived - remove from gitlab-pages

Summary

Now that gorilla web toolkit is archived (https://github.com/gorilla#gorilla-toolkit) we should assess if we keep use them or replace them by other libraries or in home-made tools. We currently have the following dependencies on https://gitlab.com/gitlab-org/gitlab-pages/blob/master/go.mod#L12:

	github.com/gorilla/handlers v1.4.2
	github.com/gorilla/mux v1.8.0
	github.com/gorilla/securecookie v1.1.1
	github.com/gorilla/sessions v1.2.0

Steps to reproduce

Example Project

What is the current bug behavior?

What is the expected correct behavior?

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Possible fixes