Refactor Broadcast class and update comments in Zulu

[scubaninja]

This pull request involves significant changes to the Broadcast class in the server/Models directory, including the addition of new properties and methods, as well as the translation of comments into Zulu. Additionally, a similar Broadcast class was added to the server/Models/tests.cs file.

Key Changes:

Updates to Broadcast class in server/Models/Broadcast.cs:

• Added several new properties to the Broadcast class, including EmailId, Status, ReplyTo, and CreatedAt. These properties include default values where appropriate.
• Translated existing comments into Zulu, providing descriptions for the new properties and methods.
• Removed the FromMarkdownEmail and FromMarkdown methods, which created Broadcast instances from markdown documents.
• Removed the ContactCount method, which calculated the number of contacts based on the SendToTag.

Addition of Broadcast class in server/Models/tests.cs:

• Added a new Broadcast class with the same properties and Zulu comments as the updated Broadcast class in Broadcast.cs.