Group Level Access Tokens / Group Bots [FE]

frontend counterpart to #214046 (closed)

Implementation plan

1. Create a HAML view similar to app/views/projects/settings/access_tokens/index.html.haml in app/views/groups/settings/access_tokens. Maybe abstract to shared partial if it makes sense.
2. Add specs similar to spec/features/projects/settings/access_tokens_spec.rb. Maybe abstract to a shared example
3. Add new nav menu item to app/views/layouts/nav/sidebar/_group.html.haml#L170
4. Copy app/assets/javascripts/pages/projects/settings/access_tokens/index.js to app/assets/javascripts/pages/groups/settings/access_tokens