Support rule-based enabling/disabling of calendars

The following discussion from !1148 (merged) should be addressed:

• @hansegucker started a discussion:

  FIXME: Feeds visible in condition of rule

assigned to @hansegucker

added board::todo part::backend type::feature labels

created branch 889-support-rule-based-enabling-disabling-of-calendars to address this issue

mentioned in merge request !1376 (merged)

closed with merge request !1376 (merged)

mentioned in commit 0c1fc129