[web] Introduce RelationshipPrompt

Summary:
Introduced RelationshipPrompt functionality for web app. It uses common logic with mobile app.
Also added basic styling, although it probably will be changed in web redesign.

Test Plan: Tested all possible cases and compared if all button actions work correctly. Checked if it works on mobile in the same way.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, benschac, atul, ashoat

Subscribers: AlisonStardust, benschac, ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D2847