[web] [fix] update join button icon with padding, adjust layout

Summary:
Update join button icon with correct size and layout now that icons have been updated

before:

after:

figma: https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/?node-id=2448%3A4361

Test Plan: look at figma and make sure button matches the design.

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3329