Add notifications for incoming mail

[RogueVoidling]

About the PR

Mail carriers(technically, anyone with a MailMetrics cartridge installed on their PDA) will now receive notifications whenever teleporters warp a batch of mail in.

Why / Balance

I just thought it would be nice for mail carriers to know when mail was warped in, so they could know when they needed to fly back and pick more up. I also added a button to toggle the notification so SRs don't have to get spammed with it, and carriers can turn it off at their preference.

Technical details

Changes to the MailMetrics: Added a new field that controls if notifications are on or off, defaulting to on. This is passed to the client whenever the state of the mail stats is updated. The server will also now listen for an event indicating notification preference has been changed, and upon receiving it, toggle the server side variable. The client side UI will also display this value, and send a message whenever its toggled.
Changes to MailSystem: Added a method that pushes a notification to everyone with the MailMetrics installed, and notifications enabled.

How to test

Spawn in a MailMetrics cartridge and install it
Use the mailnow command to spawn in mail, notice you get a notification
Toggle the notifications on and off, using the button in MailMetrics, while using mailnow to ensure the toggle works
(Remember to clear the mail teleporter so more mail can spawn!)

Media

Requirements

• I have read CONTRIBUTING.md and and am following the Pull Request and Changelog Guidelines.
• I have added media to this PR or it does not require an ingame showcase.
• I have reviewed the Ship Submission Guidelines if relevant.
• I confirm that the content in this PR is my own work, and/or is properly attributed to the original author(s).

Breaking changes

Changelog

🆑

• add: Mail carriers will now receive a toggleable PDA notification whenever mail enters the sector.