Backport authorized_keys

Closes gitlab-ee#3953

See merge request gitlab-org/gitlab-ce!16014

(cherry picked from commit 2e4a1b3b)

ac86b204 Backport authorized_keys branch 'find-key-by-fingerprint'
255a0f85 Backport option to disable writing to `authorized_keys` file
07bd79cd Combine ssh docs and rename the doc
bcffeade Use ApplicationSetting.current in Admin::ApplicationSettingsController
797fe0a6 Backport authorized_keys_enabled defaults to true'
d2f4e8f9 Avoid adding index if already exists
01319e59 Make Gitlab::CurrentSettings available when getting settings
d9557e43 Backport spec fixes in spec/lib/gitlab/shell_spec.rb
40e3d9f3 Fix typo in spec/requests/api/internal_spec.rb
bd9ead68 Fix spec in shell_spec.rb
9edd9a5e Adds changelog for backport of authorized_keys DB lookup from EE