feat(avatar): a11y changes (#2220)

[ianmcburnie]

Fixes #2220

• This PR contains CSS changes
• This PR does not contain CSS changes

PLEASE SQUASH.

Description

Having the sign in status hardcoded in the examples was problematic as the avatars are not always used for the current user.

The custom image avatar had alt text on the nested image, it makes more sense to consolidate the alt text on the root tag to be consistent across avatars.

Notes

For the default avatar, I placed the letter inside of a span element. This gives us a little more flexibility for ARIA in future (i.e. incase we decide to move the role=img and aria-label onto the nested element).

The vertical alignment in production looks a little off. I did not try and fix that in this PR.

We'll need to make some minor changes on ebayui-core side.

Screenshots

Before

After

Checklist

• I verify the build is in a non-broken state
• I verify all changes are within scope of the linked issue

• I regenerated all CSS files under dist folder
• I tested the UI in all supported browsers
• I did a visual regression check of the components impacted by doing a Percy build and approved the build
• I tested the UI in dark mode and RTL mode
• I added/updated/removed Storybook coverage as appropriate