Task scheduling interface

[Mehonoshin]

We need to provide ability to schedule when specific task will be performed.

1. Create Schedule model.
2. Add Schedule button to tasks page.
3. In the form provide ability to choose how often do we want to run required task.
4. Display scrapping schedule on the main page, in format of table with columns task name, schedule, next run, last run.

For example, the most simple case that worth to be implemented in this task is - allow to choose N hours period, when task is run.

Example: Task with id=1 should be run every 3 hours.

Later, scheduler application will receive the list of all scheduled tasks and perform them as soon as the time comes.

[Mehonoshin]

should be resolved in omniscrapper/scheduler#2