Link control needs to set aria-disabled attribute on disabled state

[davguerr]

Are you willing to submit a PR to fix? Yes
Requested priority: Normal

Describe the issue:

When Link control is set to 'disabled', aria-disabled attribute needs to be set to 'true'.
Additionally, the link (or button) should NOT be keyboard focusable on 'disable' state

Actual behavior:

1. aria-disabled attribute is not being set to any value
2. the link is focusable via keyboard

Expected behavior:

1. aria-disabled attribute should be set to 'true'
2. the link should NOT be focusable via keyboard

[lynamemi]

The PR linked above addresses #1. #2 is a broader discussion, and we are going to keep as is for now.