Add a preference to block user extensions, to allow schools to use inkscape without user extensions

This adds a checkbox to preference, which is turned off, disables the extension manager and all user extensions (anything in the user's extension directory)

This is to help schools and similar to prevent users from installing external software. It's not a pure fix, as that would require the schools to limit the directory systemically, a user could always circumvent this, but it's a barrier that might be enough.

There is a similar second design which is to move the extensions manager from the extensions menu and into the preferences. As a function you could run from there, making it less likely a student would find it.

Related issue: inkscape/extras/extension-manager#7