Fix invite member modal dropdown checkbox styling

What does this MR do?

This MR fixes the styling of the dropdown checkbox in the new invite member modal.

It also fixes that the default access level is not selected by default.

Related to #322082

Local testing

1. Enable the invite_members_group_modal feature flag
   • bin/rails console
   • Feature.enable(:invite_members_group_modal)
2. Create a project or group
3. Navigate to "Project" or "Group" -> "Members"
4. Click "Invite members"

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

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