Unbind data from user entity in favor of multisite capabilities

Currently, the server based fieldset statuses are stored with the user entity. This can lead to confusion if a user is active on multiple sites (e.g. a multisite with a shared users table) as the internal "cuids" (collapser unique ids, for form/fieldset combinations) can differ from site to site. The reason is that they are created randomly on first appearance ("first come first served").

Options:

1. Store all collapser data in a per-site table (as the cuids).

   Advantages:

   • $user->data is deprecated anyway, so this is a step towards D8.

   Disadvantages:

   • Major design change, would require a new branch.
2. Change random cuids into an md5 hash.

   Advantages:

   • No major design change.

   Disadvantages:

   • Continuing a deprecated method.

   Neutral/unclear:

   • Form states would be shared across all sites - which may be plus. Or minus.

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