Store Tracking URL in DB as well as Tracking Code

[RhodriOwainDavies]

Description

Add a new field sales_shipment_track.track_url.
Expose this field via API.
Including this URL as a hyperlink in the Shipment Sent Email template.

Feature

To support impending Magento Shipping feature request of inserting Carrier Specific URL in email template to Customer

Manual testing scenarios

1. API Test
   POST /V1/order/{orderId}/ship
   Pass in tracking_url as an extension attribute (in same manner as tracking_number)
2. UI Test
   In the admin section, view a Shipment which already has a trackinging URL set (via API in Test Can you commit to repository a folder dev/tests/static ? #1). Click Send Tracking Email. Check email, Tracking Number should be a hyperlink to tracking_url

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[orlangur]

Please continue work in #10799 instead of making a new PR with the same changes.

It would be nice to squash all changes into a single commit and force push a MAGESHIP-21 branch. Please tell me if you need any assistance with it.