[FEATURE] Prompt to set MULTIPLE_APP_MODE = true or overwrite STANDALONE_APP when installing new app

[shawncrawley]

Is your feature request related to a problem? Please describe.
This is not related to a problem, but an area where we can simplify the user experience. The latest getting started command, tethys quickstart, sets up a portal in single-app mode (i.e. MULTIPLE_APP_MODE = false in the the portal_config) with the tethysapp-hello_world application as the STANDALONE_APP. This makes a lot of sense if there is only ever going to be a single app. If the user decides to install an additional app that they want displayed in a multi-app portal, they will have to run the tethys settings command or edit the MULTIPLE_APP_MODE option directly in the portal_config. This seems like an opportunity to be "smart" and streamline the experience for the users.

Describe the solution you'd like
If a user installs another application when MULTIPLE_APP_MODE = false then as part of the install they should be prompted if they'd like to either switch to multiple app mode or swap the current standalone app with this new one.

Describe alternatives you've considered
The only alternative is to do nothing and make the user manually do this.

Additional context
We'd need to make sure that these prompts gets skipped if an install is executed with a "no prompts" options, which I'm sure we have for automated executions.

[swainn]

We should have GUI settings for the Tethys settings in the Tethys Portal admin page.

[swainn]

I like the idea of the tethys install command detecting single app mode and prompting the user if they want to turn on multiple app mode.