CRM-20799 make activity filter user preference an optional setting

In CRM-11761 we added a feature where the include/exclude filters on the activity tab were saved as user preferences. when a user set a filter, upon returning to any activity tab, that existing filter was preserved and reapplied.

that behavior should really be optional – handled by a system setting – rather than enforced on all users.

1. add setting to Customize Data and Screens > Display Preferences. insert below "Include ICal Invite to Activity Assignees". Label is: Preserve activity filters as a user preference. Help text: When enabled, any filter settings a user selects on the contact's Activity tab will be remembered as they visit other contacts.
2. condition the storing and retrieving of the user preference based on this setting