Split Mailings into separate tab if activity creation is disabled

1. add a option in CiviMail preferences - "Enable CiviMail to create activities on delivery"

• if unchecked we should not create activities for "Bulk email" using CiviMail
• also enabled "Mailings" tab automatically if above option is unchecked.

2. create new "Mailings" tab that will display mailings for a particular contact.

By default mailings tab is disabled.

schema and data changes

• add new option_value for contact_view_options group - "Mailings"
• also handle upgrade