Document alert message template system

[lunkwill42]

Describe the bug

The fact that alert message texts (notifications) in NAV are configurable isn't documented properly anywhere, except for two, non-obvious mentions:

• The release notes for NAV 3.13
• An aside in the MailIn subsystem documentation

What is documented is the similar event templates system, which is solely for presentation in the NAV web UI, and has nothing to do with notifications:

• Event Templates

Expected behavior

The location, structure, format and purpose of alert message templates should be documented. Things to think of:

• How they are filled by event engine at the time of alert generation (therefore retroactive changes aren't "possible")
• That they can be easily modified
• That multiple languages are inherently supported, even though only english templates are supplied with NAV.
• Explain the difference between these templates and event templates (as mentioned above)