Give modal with instructions to install desktop client

[quinthar]

Right now when you go to install the desktop client, you get an error message and it's very unclear how to get around it. I expect like 90% of employees who try will get this error, and not know how to fix it. Let's just anticipate that and when you click "Desktop" we show a message:

To install:

1. Download this link
2. Open it
3. If you get a "cannot be opened because the developer cannot be verified" error:
   1. Hit Cancel
   2. Go to System Preferences > Privacy > General
   3. Click Open Anyway at the bottom
   4. When it re-prompts you, click Open

Assigning MED because nearly everyone who tries to use the desktop client will experience this

[AndrewGable]

It looks like modals aren't supported on react-native-web yet: necolas/react-native-web#1020

But alert() is supported:

While it doesn't look great, it gets the point across. Does that sound OK?

[quinthar]

Yep, better than nothing