The calendar_project_tasks and calendar_user_tasks settings were used as a
column name in the iCal query, and escapeIdentifier() leaves values with a
space or a dot untouched. A stored value like "1 OR 1=1 OR 1" made the public
project feed return tasks from every project.
Only date_creation and date_started are accepted now, anything else falls
back to date_started. The currency form also stops saving keys other than
application_currency, which is how the value could be written in the first
place.
Fixes#5904
