Allow custom favicons

[richard-to]

If static folder is enabled, then we will check if there is a favicon.ico in the designated static folder. If it exists, we will use the user's favicon instead of the default.

Also:

• Moved the static folder functions to server_utils.py
• Improved validation slightly for static URL paths
• Updated docs with custom favicon example

Closes #563

For a future improvement, we can add more complete favicon support. See https://www.emergeagency.com/insights/detail/the-essentials-of-favicons/