Improve the email input experience within the invite modal

What does this MR do and why?

Implement the MVC solutions in #430089 (closed):

1. When user is inputting something and the dropdown shows suggestions, pressing the tab key should do the same thing as pressing the enter key to select it. This is a common UX pattern we should be utilizing, similar pattern in Gmail demonstrated in this loom video.
2. Update the form field title to "Username, name or email address" to reflect that the field also searches by user's first name and last name.

How to set up and validate locally

1. When in a group or project, click the "+" icon in the top right of the sidebar and select "Invite team members".
2. Interact with the "Username, name or email address" search field to observe.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #430089 (closed)