Methods are replaced to computeds in invite_members

What does this MR do and why?

Cleaned up invite_members_trigger.vue, methods are replaced to computeds

Screenshots or screen recordings

project_members_page

mr_page

I couldn't find the component using in the dropdown-mode (3/5 prop variations are in recordings)

How to set up and validate locally

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 #408473 (closed)