Increase accessibility of Invite Members modal

What does this MR do and why?

1. Include a high-level form error that gets focused on submit when member invites are empty.
2. The next focusable element is the invalid input.
3. The invalid input is marked as such and has the error styles. (current behavior)
4. The help text on the invalid input should be replaced with text to remedy.
5. Invite modal inputs given accessible names with labels.

Screenshots or screen recordings

GlAlert on empty invite submit

UI

Before	After

Windows / Chrome / JAWS

Before	After
jaws-invite-members-modal-before	jaws-invite-members-modal-after

Windows / Firefox / NVDA

Before	After
ndva-invite-members-modal-before	ndva-invite-members-modal-after

MacOS / Safari / VoiceOver

Before	After
invite-members-modal-before	invite-members-modal-after

How to set up and validate locally

1. Visit group or project member pages e.g.
   • /groups/groupName/-/group_members
   • /groupName/projectName/-/project_members
2. Press "Invite members" button
3. Submit form without inviting members
4. Should display move focus to alert
5. Next focusable element is the search field with error state and message
6. Inputs have accessible names via labels (labels can be clicked to trigger input focus)

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.

Closes to #371262 (closed)