Do not use absolute URLs in illustrations

[hjalves]

This fixes #6704.
Works in both non-root app context and root app contexts.

Before submitting the PR, please make sure you do the following

• If there are changes to the backend codebase, run the backend tests with lein test && lein eastwood && lein bikeshed && lein docstring-checker && ./bin/reflection-linter
• Run the frontend and integration tests with yarn lint && yarn flow && yarn test)
• Sign the Contributor License Agreement
  (unless it's a tiny documentation change).

[tlrobinson]

Thanks for doing this. Looks like there's a few others if you feel like handling those, otherwise we can do another PR:

$ grep -r 'image="/' frontend
frontend/src/metabase/containers/EntitySearch.jsx:                  image="/app/img/empty_question"
frontend/src/metabase/dashboards/containers/Dashboards.jsx:              image="/app/img/dashboard_illustration"
frontend/src/metabase/dashboards/containers/Dashboards.jsx:                  image="/app/img/empty_dashboard"
frontend/src/metabase/dashboards/containers/DashboardsArchive.jsx:                  image="/app/img/empty_dashboard"
frontend/src/metabase/new_query/containers/MetricSearch.jsx:                  image="/app/img/metrics_illustration"
frontend/src/metabase/new_query/containers/NewQueryOptions.jsx:                    image="/app/img/questions_illustration"
frontend/src/metabase/new_query/containers/NewQueryOptions.jsx:                  image="/app/img/query_builder_illustration"
frontend/src/metabase/new_query/containers/NewQueryOptions.jsx:                    image="/app/img/sql_illustration"
frontend/src/metabase/questions/containers/QuestionIndex.jsx:      image="/app/img/questions_illustration"

[hjalves]

Ok, I will update this MR with those changes after making some tests

[hjalves]

Tested every page where those illustrations appear. Both with non-root and root URL's.

[kdoh]

Thanks @hjalves!