Rework Mail System

[JRoy]

1. New /mail sendtemp <time diff> <message> command to send mail that will self-destruct after time diff.
2. New /mail clear <number> command to clear a specific mail item.
3. /mail read now tracks which mails you read and won't nag you about them.
4. A bunch of other flexibility since we store actual data instead of strings

No longer relavant json format since new format will serialize to YAML using configurate object mappers

Mail Schema:

{
  "mail": [
    {
      "read": false, //bool which if true will show the mail item as gray italic to the user when read otherwise white
      "sender": "JRoy", //String because fetching User objects sounds like a bad idea
      "uuid": "abcde-abcd-acd12-12334", // Not currently used but stored for possible future use of for a server admin's reference
      "timestamp": 1234567890, //Epoch allows timestamp format to be changed (or per-player timezones in the future)
      "expire": 1234567890, //Epoch at which this mail should be deleted and not shown to the user or 0 if it does not expire
      "message": "Cool message, yo" //Actual message, possibly with color codes
    },
    {
      "read": false, //bool which if true will show the mail item as gray italic to the user when read otherwise white
      "legacy": true, //bool but Value of this doesn't matter as it just instructs the parser to not expect the other *unknown* values
      "message": "JRoy: this is a message that was sent before fancy system" //Actual message
    }
  ]
}

Closes #364
Closes #1527
Closes #1860
Closes #3097

[JRoy]

Moving to draft as this is pending #4072 being merged

addressed