Recent changes in both Magic-Folder and ZKAPAAuthorizer have broken compatibility with older Tahoe-LAFS and Gridsync configurations: Specifically, the "magic-folder" feature present in earlier versions of Tahoe-LAFS has been removed as of Tahoe-LAFS version 1.15 (having been split off into a standalone project with a different configuration format), while ZKAPAuthorizer's newer backup/restore system has removed the older "v1" endpoints used by previous versions. Unfortunately, neither project has provided a migration path for converting an older configuration to a new one, thus leaving on-disk configurations that cannot be used with newer versions of the software.

This PR checks for such out-of-date configurations within the Gridsync configuration directory and, if found, displays a message to the user, explaining the compatibility-break and providing suggestions for how to continue.