Add modal for TCP apps in Web UI

[ravicious]

Today, TCP apps in the Web UI are not usable. They are listed with other resources, but they cannot be interacted with. The Web UI shows a disabled Launch button with some extra text on hover.

To address this, let's change the button to say “Connect” (“Launch” makes sense only in the context of web apps, not TCP apps). Clicking on it would bring up a modal similar to the one which we show today for databases and kubernetes clusters:

On platforms where VNet is supported (Windows, macOS) the modal for TCP apps should show a primary button with a deep link to Teleport Connect. That link would open the VNet marketing tab in Teleport Connect. Until the marketing tab is implemented, this should simply open the VNet panel without starting VNet. There would also be a secondary button with a download link.

After that, the modal would show how to use tsh proxy app to connect to the app without VNet. If the platform does not support VNet, the modal shows only that information instead.

• Add modal for TCP apps.
• Implement VNet deep link in Connect.
• Investigate if modal for cloud apps is needed. If so, add it.

[ravicious]

I'm dropping the idea of adding a modal for cloud apps. It looks like both Azure and Google Cloud apps would need their own instructions and I don't have access to any real apps to test that. When I was writing this issue, I was hoping that it'd be a matter of adding a single, simple modal, but that's not the case.

What's more, it turns out that cloud apps are treated as TCP apps in Connect which is wrong. I'll create a separate issue for improving support for cloud apps in our GUIs.