Remove `eye` icons from Confidentiality in sidebar and add `eye-slash` icon to the confidential banner

Context

Following up on gitlab-ui#1813 (closed)

We need to ensure the alert used in the Confidentiality row within the sidebar conforms with Pajamas. Currently it is lacking an icon.

Proposal

• Add the eye-slash icon to the current alert to better conform with Pajamas
• Remove the eye (when not confidential) and eye-slash (when confidential) icons from the Confidentiality sidebar item

When confidential

Before	After

When not confidential

Before	After