So that we don't need to hold env after the request.
This makes it much harder to test, especially Rails session is
acting weirdly, so we need `dig('flash', 'flashes', 'alert')`
to dig the actual flash value.

since `Rails.application.routes.url_helpers` creates a new anonymous module every time it's called

In !15082, we changed the behavior of the middleware to call
`Rails.application.routes.recognize_path` whenever a new route arrived.
However, this can be a CPU-intensive task because Rails needs to allocate
memory and compile 850+ different regular expressions, which are complicated
in GitLab.

As a short-term fix, we can do a lightweight string match before
we do the heavier comparison.

Closes #40185, gitlab-com/infrastructure#3240

- Also remove sign_out DELETE route from read-only whitelist routes

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.