Introduce Credentials Inventory

app/finders/keys_finder.rb

app/finders/personal_access_tokens_finder.rb

app/models/key.rb

app/models/personal_access_token.rb

app/views/layouts/nav/sidebar/_admin.html.haml

db/migrate/20191205060723_add_index_to_keys.rb

db/schema.rb

doc/administration/compliance.md

doc/administration/index.md

doc/user/admin_area/credentials_inventory.md

doc/user/admin_area/img/credentials_inventory_v12_6.png

ee/app/controllers/admin/credentials_controller.rb

ee/app/helpers/admin/credentials_helper.rb

ee/app/models/license.rb

...views/admin/credentials/_personal_access_tokens.html.haml

ee/app/views/admin/credentials/_ssh_keys.html.haml

ee/app/views/admin/credentials/index.html.haml

...s/personal_access_tokens/_personal_access_token.html.haml

ee/app/views/admin/credentials/ssh_keys/_ssh_key.html.haml

ee/app/views/layouts/nav/sidebar/_credentials_link.html.haml