Remove dependency on Echo

[htdvisser]

Summary

This issue tracks the removal of our dependency on Echo. We've already done some work, but we didn't have an issue to track it.

Why do we need this?

#2382 (comment)

What is already there? What do you see now?

A lot of work already done in #2382 and #2561

More upcoming removals in #2564

Some other pull requests removing other parts

What is missing? What do you want to see?

Complete removal

[htdvisser]

Let's add this to the quarterly milestone. We haven't been able to upgrade Echo since v4.1.2 (released Apr 29, 2019) which is a really long time. At the same time, we don't actually need Echo, because we also use Gorilla mux router, so let's just remove what's left of Echo.

[johanstokking]

So we have:

• pkg/account, pkg/oauth, pkg/web/oauthclient: @kschiffer
• pkg/basicstation/cups: @KrishnaIyer
• pkg/console and pkg/webui: @pgalic96 (touched these files last week and usage is very minimal)
• pkg/gatewayserver/io/ws: @KrishnaIyer
• pkg/web/middleware: @pgalic96 (needs non-echo variants)

General cleanup, may depend on the above (?)

• pkg/errors/web: @htdvisser
• pkg/ratelimit: @htdvisser (this is probably just deleting the middleware that has become obsolete)
• pkg/util/test/assertions: @htdvisser (independent; the test assertions aren't being used)
• pkg/web: @htdvisser and/or @johanstokking (definitely depends on removing usage above)

[htdvisser]

pkg/web/middleware: @pgalic96 (needs non-echo variants)

Those should all already exist in pkg/webmiddleware.

[adriansmares]

* `pkg/basicstation/cups`: @KrishnaIyer
* `pkg/gatewayserver/io/ws`: @KrishnaIyer

I'll steal this.