[web] Allow displaying multiple modals

Summary:
Modified Modal API to allow displaying multiple modals in a stack.
Used array indexes as keys for rendering array of modals, as they never changes between renders - we only push or pop modals from the stack.
Linear: https://linear.app/comm/issue/ENG-967/allow-displaying-multiple-modals-in-a-stack

Test Plan:
Tested the feature by adding temporary an action displaying confirmation modal after clicking on member in MembersModal.
Try displaying all existing modals to confirm, their behavior remain unchanged after introducing modal API changes.

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3665