GP-15420: Add task scheduling.

[awg-github]

General

1. Added new schedule_start_date column to the civicrm_sqltasks table. It adds in upgrade_0160.
2. Canged retrieving tasks to the dispatcher(triggered by a scheduled Job).
   Before in WHERE statment:
   enabled = 1
   Now:
   enabled = 1 AND (schedule_start_date < NOW() OR schedule_start_date IS NULL)

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'Configure task' page

1. Added 'task execution schedule' to the execution block.
2. When user change any option it updates schedule.
3. Removed 'day' dropdown when is checked "weekly" on "Execution frequency" dropdown.
   

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'Manage tasks' page

1.When user click on 'enable task' button it show dialog where user can set 'next execution schedule date' and can see the schedule.
2. When user click on 'confirm' button it will save 'execution schedule date' to task and will anabe task.

[pfigel]

@awg-github string for next cron: "Next cron task execution"