Provide a mechanism to log submission transactions

Follow-up to #2854017: Provide a handler to schedule the sending of emails

Problem

Webform handlers execute transactions that should be optionally tracked with their results. For example, when a scheduled email is sent, the send date and time should be tracked to prevent duplicate emails. Another use case, would be the response from a remote post should be logged and tracked

Solution

Create a table to optionally log webform handler transactions.

Tasks

• Add 'webform_submission_log' table
• Update WebformSubmissionStorage class
• Add log tab to submissions.
• Only users with 'administer submission' permission can view logs.
• Update TestHandler to log all requests.
• Write tests

Database Changes

webform_submission_log

• lid => 1 (key)
• sid => 1
• uid => NULL|1
• type => scheduled_email
• operation => cron
• value => sent
• timestamp => 1487024261
• indexes: [sid] and [type, operation, value]