[Improvement] SMS subscriber validation is required for all new subscribers

[Improvement] Added ability to change CSS styles for subscriber links (RSS, iCalendar and Twitter)

[Improvement] All API errors are sent as JSON

[Bug] Fixed SMS unsubscribe issue causing some subscribers to remain listed as a subscriber

[Improvement] New notification subscribers automatically subscribe to updates for all new components and containers

[Improvement] Updated the variable names for plain-text mail notification templates

[Bug] Fixed bug calculating incorrect days until next maintenance

[Improvement] Split up the HTML head and body for custom code

[Bug] Fixed bug when displaying invoices

[Improvement] Backend performance optimizations

[Bug] Fixed bug causing incident update view to reload if a component was deleted