add support for ignoring specific folders under the assets directory

[CNFeffery]

The existing parameter assets_ignore can ignore files under the assets folder that match a specified regular expression pattern, but it cannot ignore specific folders under the assets folder as a whole, which would be useful in many scenarios.

[gvwilson]

thanks for the suggestion @CNFeffery - we're trying to get the Dash 3.0 release out the door right now, but I'll see if we can review #3077 soon after.

[BSd3v]

@CNFeffery ,

Have you used the static (flask default) folder to keep things from being uploaded to the server?

[CNFeffery]

@BSd3v I hope the handling method in this scenario is as simple as possible, so I added a parameter that is convenient, fast, and easy to understand.