[Rails 5] Fix link_to_member_avatar

What does this MR do?

This is a quick fix for

    ActionView::Template::Error:
       nil is not a valid asset source
     # ./app/helpers/lazy_image_tag_helper.rb:12:in `image_tag'
     # ./app/helpers/projects_helper.rb:27:in `link_to_member_avatar'
     # ./app/helpers/projects_helper.rb:39:in `link_to_member'
     # ./app/views/shared/issuable/_participants.html.haml:14:in `block in _app_views_shared_issuable__participants_html_haml___1967790359849897399_307108740'
     # ./app/views/shared/issuable/_participants.html.haml:12:in `each'
     # ./app/views/shared/issuable/_participants.html.haml:12:in `_app_views_shared_issuable__participants_html_haml___1967790359849897399_307108740'
     # ./app/views/shared/issuable/_sidebar.html.haml:122:in `block in _app_views_shared_issuable__sidebar_html_haml__4258134513421332557_279739320'
     # ./app/views/shared/issuable/_sidebar.html.haml:18:in `_app_views_shared_issuable__sidebar_html_haml__4258134513421332557_279739320'
     # ./app/views/projects/issues/show.html.haml:91:in `_app_views_projects_issues_show_html_haml__4363779578181523868_325530840'
     # ./lib/gitlab/i18n.rb:47:in `with_locale'
     # ./lib/gitlab/i18n.rb:53:in `with_user_locale'
     # ./app/controllers/application_controller.rb:330:in `set_locale'
     # ./lib/gitlab/middleware/multipart.rb:93:in `call'
     # ./lib/gitlab/request_profiler/middleware.rb:14:in `call'
     # ./lib/gitlab/middleware/go.rb:17:in `call'
     # ./lib/gitlab/etag_caching/middleware.rb:11:in `call'
     # ./lib/gitlab/request_context.rb:18:in `call'
     # ./config/initializers/fix_local_cache_middleware.rb:9:in `call'
     # ./lib/gitlab/middleware/static.rb:9:in `call'
     # ./lib/gitlab/testing/request_blocker_middleware.rb:36:in `call'
     # ./lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'
     # ------------------
     # --- Caused by: ---
     # ArgumentError:
     #   nil is not a valid asset source
     #   ./app/helpers/lazy_image_tag_helper.rb:12:in `image_tag'

I think the method itself should be refactored in upstream (gitlab-ce):

1. Use one parameter (lazy) instead of two (lazy and lazy_load).
2. Don't call avatar_icon which goes to the database (in some cases) if we have lazy:true.
3. Pass lazy parameter to the image_tag from the link_to_member_avatar.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12841