feat: Install PWA button

[xgraceyan]

Summary

• Adds an "Install App" button to the Header for easy PWA installation.
  

• Button changes to just icon in mobile view, to ensure everything in the header fits

• Button disappears if PWA is unsupported (Safari, IE?) or if the user has the app already installed.

• Added PWA store to preserve PWA install prompt (the install prompt was lost upon desktop to mobile window resize when only storing it to component state)

Test Plan

• Ensure button prompts + installs PWA properly on both desktop and mobile
• Ensure buttons in the header all fit on mobile view (width <400px)
• Ensure button disappears if PWA is already installed, on both the web and PWA versions.
• Ensure button reappears upon PWA uninstallation
• Ensure button is hidden when user is on unsupported browser (Safari & Firefox for desktop, IE -- if anyone uses that anymore)

Issues

Closes #936

[MinhxNguyen7]

Seems to work from a quick check on my phone. FYI, the check allows the button to hide after the PWA has been installed or if you're in incognito.

That being said. I think a banner, or just somewhere other than the top bar, might be better.

[KevinWu098]

Seems to work from a quick check on my phone. FYI, the check allows the button to hide after the PWA has been installed or if you're in incognito.

That being said. I think a banner, or just somewhere other than the top bar, might be better.

I agree here -- we have pretty limited space in the header on smaller screen sizes. A dismissible banner (not unlike RecruitmentBanner) might be better for grabbing user attention w/o compromising on permanent screen real estate

[MinhxNguyen7]

But then, we agreed that people aren't likely to install it before actually using it, so they'd just dismiss it.

How about a banner that comes back if it has been long enough? Something like a one-month delay could make sense.

[KevinWu098]

Makes sense @MinhxNguyen7!