Summary:
Shares and documents would both produce a rule set for the same column
if the document rule set was for multiple columns. In this case, it was causing
one of the rules to be overwritten by the other (specifically, the rule granting
access to form references was not being applied in shares). The symptom was
`null` values in place of the referenced table's values.
We address this by splitting any rule sets for multiple columns that are also
affected by shares, so that they can be overridden by shares without causing a
conflicting rule set to be created (i.e. 2 column rule sets containing the same column).
Test Plan: Server tests.
Reviewers: dsagal, paulfitz
Reviewed By: dsagal, paulfitz
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D4208
