fix: the display of username suggestions

[khudym]

Description

Username suggestions functionality has been corrected to ensure proper behaviour.

• fixed issue with suggestions being displayed in the wrong place;
• added the ability to navigate through suggestions using the keyboard.

Related Pull Requests
PR to the open-release/quince.master branch: #1278
PR to the master branch: #1270

Steps to Reproduce:

1. suggestions being displayed in the wrong place

Open register page -> Enter some string into field "Full name" (e.g. Jack Smith) -> Click on field "Public username"

suggestions_rendering_bug.mov

2. impossible to enter suggestions field using the keyboard

Open register page -> Enter some string into field "Full name" (e.g. Jack Smith) -> press Tab button until you get to the "Public username" field

suggestions_keyboard.mov

After changes:

1. suggestions being displayed in the wrong place

fixed_suggestions_rendering_bug.mov

2. impossible to enter suggestions field using the keyboard

fixed_suggestions_keyboard.mov

Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Sandbox, if applicable.
• Is there adequate test coverage for your changes?

Post-merge Checklist

• Deploy the changes to prod after verifying on stage or ask @openedx/2u-vanguards to do it.
• 🎉 🙌 Celebrate! Thanks for your contribution.

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