Modal UI and button for inviting other admins

## Description
As an Admin,
I am able to invite other admins via a modal UI that pops-up by clicking on a button.

## Mocks
*Need to be created*

## Acceptance Criteria
### Update [Required]
- [ ] Field to input email for the invited user
- [ ] Field to invite as: User or Admin

## Definition of Done
- [ ] All of the required items are completed.
- [ ] Approval by 1 mentor.

## Estimation
3 hours