Resolve "Make Incident/Maintenance external reference cliquable"

What does this MR do and why?

Incident and Maintenance now have a new attribute external_link. It allow the user to add a cliquable link to access the resource in an external service.

This new attribute add a link on the external_reference badge in the resource page

This new parameter is configurable with the standard CREATE and UPDATE form

Screenshots or screen recordings

MR acceptance checklist

• My code follows the style guidelines of this project
• I ran pylint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)
• I have added user readable comment in the CHANGELOG

Closes #80 (closed)