Reduce the number of skip links and use AUI implementation

Current result:

There are five skip links available in the top nav, and they're invisible on focus. 

Expected result:

1. As per the testing team suggestion (and also reported by customers), we should reduce the number of skip links for logged users to two:

• Skip to sidebar (if the sidebar is available on the screen)
• Skip to main content

2. Skip links should be visible, using the available AUI component.

https://aui.atlassian.com/aui/latest/docs/page.html 

3. When user is not logged in, we should display an additional skip link on the first position

• Log in

and then

• "Skip to sidebar"
• "Skip to main content"