Move Relay icon tab activation order to *before* the input field

[Vinnl]

Current behaviour:

• press Tab to focus an email input field
• press Tab again - the Relay icon gets focus
• press Tab again - the password input field gets focus

Requested behaviour:

• press Tab to focus the Relay icon attached to an input field
• press Tab again to focus the email input field
• press Tab again to focus the password field

There are two benefits to this:

1. The Relay icon menu isn't very useful if you've already entered something into the email field.
2. People have muscle memory of pressing Tab from an email input field to get to the password field.

The downside is that the visual order no longer corresponds with the tab order.

Context: https://github.com/mozilla/fx-private-relay-add-on/issues/77#issuecomment-1823893101

[Vinnl]

Another report here: https://addons.mozilla.org/en-GB/firefox/addon/private-relay/reviews/2023546/

(Although that person might also already be satisfied by just disabling the icon.)

[onebod]

I would say desired behaviour: as previously normal and expected - active field: username, tab to get to password field. shift+tab to get to relay icon if it were placed as you suggest - left of username field.