Improve how we handle incompatible macros coming from server

Issue Summary

There is a myriad of macros that can be used in the Confluence Server/DC that are not compatible with the Cloud environment.

When you migrate a space that contains pages with macros that are not compatible with Cloud the macro breaks and start showing a warning box with "unknown macro":

An example:

Expected Results

The incompatible macros code would be stripped during import, leaving the text part inside the macro intact.

Actual Results

A warning box is shown saying the macro is unknown.

Workaround

You can remove the macros on the server prior to migrating to the Cloud.