[web] Add modals for updating the desktop app

Summary:
Adds modals for updating the desktop app. There are two cases:

• The user is using the first version of the app. We check if the version string exists and if not, we prompt the user to download manually a new (self-updating) version. This is shown on refresh.
• If the user is on the self-updating version, after downloading the new version we ask if they want to restart the app

(both of them only show up when it's the desktop app not the web app)

More info here: https://linear.app/comm/issue/ENG-2661/

Test Plan:

• Check if the user is prompted to download the new version if there is no version string
• Check if the user is prompted to restart after a new version is available and the update is applied correctly

Reviewers: ginsu, przemek, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6324