Audit components and ensure secure links are generated

[jportner]

Currently, if you use EuiLink to create a link, it implements a security control that uses the href property to calculate the appropriate rel attribute (added in #337). This is needed to ensure that potentially malicious external websites cannot modify the DOM of the source page.

However, other components that support the href property (such as EuiBadge and EuiListGroupItem) do not set the rel attribute. We should audit all of the EUI components to find what gaps exist, and ideally put a linter rule in place to prevent regressions in the future.

[chandlerprall]

To address, we need to:

• automate detection of these instances (e.g. eslint rule), use it to audit existing code and prevent regressions
• fix detected instances

[Marvin9]

@chandlerprall Can work on it?

[chandlerprall]

Resolved by #3272