Make install command smartly prompt user if MULTIPLE_APP_MODE is False

[shawncrawley]

Description

This merge request implements feature #1143

Changes Made to Code

• Core logic of this new feature was added to tethys_cli/install_commands.py
• New tests written

Related PRs, Issues, and Discussions

• [FEATURE] Prompt to set MULTIPLE_APP_MODE = true or overwrite STANDALONE_APP when installing new app #1143

Additional Notes

Quality Checks

• At least one new test has been written for new code
• New code has 100% test coverage
• Code has been formatted with Black
• Code has been linted with flake8
• Docstrings for new methods have been added
• The documentation has been updated appropriately

[coveralls]

coverage: 100.0%. remained the same
when pulling a1f7730 on smart-scaffold-detect-app-mode
into 110efb4 on main.

[shawncrawley]

This was one I had gotten essentially 95% done in the last months and then just left it by the wayside. Not essential for this next release, but couldn't hurt either.

[shawncrawley]

This one should be good to go. Merry Christmas!