Add some kind of flood-control

[jschreuder]

Add limitations through configuration (should be high enough not to cause trouble for normal usage):

• [ ] Maximum number of tickets imported per hour (default: 250)
• [ ] Maximum number of tickets created by single e-mail address per day (default: 25)
• [ ] Maximum number of replies added by single e-mail address per day (default: 250)
• [ ] Send rate-limited user single notification when limit is reached
• [ ] Decide how & if to apply limits to users