Add and remove documentation and link for dataset

[kkorchak]

Datased documentation test steps:

1. Open test dataset.
2. Edit documentation and save, verify documentation saved correctly
3. Remove documentation, verify documentation removed.
4. Return documentation to previous state
5. Add documentation link, verify URL warning and dialog.
6. Remove documentation link, verify link removed.
7. Return documentation link to previous state.
8. Edit and save field documentation(Schema tab)
9. Return field documentation to previous state.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[jjoyce0510]

looks great!