Filesystem performance can have a big impact on overall GitLab
performance, especially for actions that read or write Git
repositories. This information will help benchmark filesystem
performance against known good and bad real-world systems.

We remove this feature as it never worked properly

Why and how to enable this is covered in the docs being changed
here. This requires gitlab-org/gitlab-shell@2e8b670 ("Add support for
SSH certificate authentication", 2018-06-14) which has been merged in
and tagged as 8.0.0, so GITLAB_SHELL_VERSION needs to be bumped.

Merging this closes gitlab-org/gitlab-ce#34572 see
gitlab-org/gitlab-shell!207 for the gitlab-shell MR.

Implementation notes:

 - The APIs being changed here are all internal, and their sole
   consumer is gitlab-shell.

 - Most of the changed code is a MR to gitlab-shell, see the
   gitlab-org/gitlab-shell!207 MR. That change covers why only some of
   the internal methods get a new "username" parameter, and why some
   others only get a "user_id".

Backport to CE, originally branch 'evn-ssh-clarify-docs

See merge request gitlab-org/gitlab-ee!3753

Originally branch 'mk-toggle-writing-to-auth-keys-1631'

See merge request !2004

Squashed commits:
Add authorized_keys_enabled to Application Settings
Ensure default settings are exposed in UI
Without this change, `authorized_keys_enabled` is unchecked when it is nil, even if it should be checked by default.
Add “Speed up SSH operations” documentation
Clarify the reasons for disabling writes
Add "How to go back" section
Tweak copy
Update Application Setting screenshot