Tutorials about migrating from User CustomAction, ECB, JSLink to SPFx extensions (SharePoint#1023)

* Removed reference to Visual Studio, which is not valid anymore.

* Fix for type in the hosting-extension-from-office365-cdn.md article

* Added tutorial about migrating from CustomAction to Application Customizer.

* Added tutorial "migrate-from-ecb-to-spfx-extension"

* Added tutorial about migrating from JSLink to Field Customizer.

* Small fix to the ECB custom extension

Author: PaoloPia