Schedule follow-up activity when adding or editing an activity AND provide link to "Prompted by" activity when viewing a follow-up activity

Summary
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Give users the ability to schedule a follow-up activity while they are adding or editing an activity record.

NOTE: This feature is desirable for both Case activities and "core" activities. However, depending on decisions about integrating the code for core and case activities - it may be available only for activities in a case in the 2.2 release.

Implementation
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1. New fieldset added at the bottom of the Activity form. Fieldset should be collapsed by default. Legend / fieldset title = "Schedule Follow-up"

2. Fieldset has 2 fields contained in a sentence:

"Schedule Follow-up <$select_activity-type> in [ $interval ] <$interval_unit>."

• $select_activity_type is a jQuery autocomplete select which allows user to select any valid activity type for the given Case Type. This should be the same field / behavior as the "New Activity" field in View Case EXCEPT that we set a default activity type = activity type of currently viewed activity. (For "core" activities - the selection set for this field is ANY "core" activity type).
• $interval is a text field
• $interval_unit provides options: day(s), week(s), month(s), year(s) - default value is day(s).

3. Form Validation

• if an activity type is selected then interval is required, and interval_unit is required
• If interval is not empty, it must be an integer value and activity type and interval_unit are required

4. Post Process:
If "schedule follow-up" fields are present in params, after insert / update operation for the "main" activity:

• Retrieve civicrm_activity.id of newly inserted "main" activity record - this will be the parent_id for the follow-up
• Insert follow-up activity record with the following values:

• activity_type = the selected follow-up activity type
• parent_id = id of the "main" activity record
• status = "Scheduled"
• due_date = calculated from now() based on interval and interval_unit, set timestamp part to current time
• is_auto = 0

5. Provide link back to parent activity when viewing a follow-up activity

• Modify ActivityView to check if parent_id is NOT NULL
• If NOT NULL, assign $parentURL to template - this should be a link to view the "parent" activity. (ActivityView.tpl already has logic to display this link if it's got a value).

"Prompted by..."