We set `SameSite=None` in GitLab 12.10 via
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/28205 because
Chrome v80, rolled out in March 2020, treats any cookies without the
`SameSite` directive set as though they are `SameSite=Lax`
(https://www.chromestatus.com/feature/5088147346030592). This is a
breaking change from the previous default behavior, which was to treat
those cookies as `SameSite=None`.

However, older browsers (e.g. MacOS 10.14 on Safari 13.0.3) may
interpret the `None` as `Strict`, which causes users that click on
gitlab.com links from third-party sites (e.g. Gmail, Slack) to log in
again.

https://www.chromium.org/updates/same-site/incompatible-clients
recommends a set of regular expressions to determine whether to send
this. This commit implements most of the logic but skips one case since
this doesn't seem common: macOS 10.14 with an embedded WebKit
browser. This is also what
https://rubygems.org/gems/rails_same_site_cookie does.

I considered using that gem
(https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40663), but I
didn't like how it added another dependency (`user_agent_parser`) that
loads a large YAML database
(https://github.com/ua-parser/uap-ruby#the-pattern-database).