Enable sieve redis MTA option

Sieve language extension MTA options: (0-2)
New in the 8.0.2.3 release. The  MTA option controls whether Sieve filters may use the   operator to access and manipulate stored data using the redis protocol. A value of  disables the use of   in all Sieve scripts. The default value is, meaning that   may be used in any Sieve script. A value of  allows    to be used in system-level Sieves only.

When  is set to ,  then any attempt to use   will result in a Sieve error  "Redis access has been disabled".

When  is set to , then  user-level attempts to use   will result in a Sieve error  "Redis only allowed in system-level sieves".

See also:
 * Sieve filters
 * Sieve redis extension
 * Redis MTA options
 * Sieve language extension MTA options