Activity Search with support for Export (including custom fields) and Batch Update via Profile

Rationale
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Users need built-in Activity search functionality which allows them to search on activity custom fields and export activity data (including custom activity field data).

Implementation
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Create a new 'Find Activities' search interface accessed via a new URL and navigation menu item (in Search menu) at civicrm/activity/search. The search criteria will be based on the criteria currently available in Advanced Search -> Activities pane, but with some improvements noted below. As with other 'component' searches, the same (improved) criteria should be exposed to Advanced Search.

1. Search criteria - additions and modifications to the existing Activity search criteria (CRM_Activity_BAO_Query::buildSearchForm):

• Activity Type is now multi-select instead of single select dropdown (use the 'listing-box' UI - rows of checkboxes - for this, as done w/ Groups and Tags in Adv Search)
• Search by custom field:
  • Searchable activity custom fields which extend ALL activity types are always displayed in the form.
  • Searchable activity custom fields which extend specific activity types are injected into the search form dynamically based on the user selecting specific Activity Type(s) to search on.

2. Search results columns:
I think the columns in the existing core Activity selector s/b fine: Type, Subject, Added By, With, Assigned, Date, Status + action links (permission dependent): Edit, View, etc.

3. Search results tasks ('actions'):

• Export:
  • Primary field export includes basic contact data (e.g. name, address .... same set of fields as other components include - eg. contribution search), plus all core activity fields, plus all custom fields defined for activities (all types and specific types).
  • Select fields (mapped export) also supported
• Delete Activities
• Send Email to Contacts - NOTE: I think this task will need an intermediate step for user to select whether they want to send to With, Assigned to AND / OR Created by contacts (checkboxes so they can select one or more of these) - default s/b 'With' (e.g. target contacts).
• Batch Update Activities via Profile

4. Implement Batch Update via Profile for Activities

• Allow user to create a profile of type = Activity by adding core and custom activity fields
• This type of profile ONLY allows activity fields
• Batch update grid exposes read-only columns as specified in 'Autocomplete' preferences (same as other component-based batch updates)