[Admin] Use an accessibility linter for the new admin

[waiting-for-dev]

We want to build a new admin which is accessible to anyone. We can perform a simple task toward that goal using an accessibility linter.

In quick research, axe looks like a good candidate. We would probably need both axe-core-capybara & axe-core-rspec. However, it could be possible to find a better tool, so let's also do a bit more of investigation.