v1.24.0

This release of the ntfy server brings supporting features for the ntfy iOS app. Most importantly, it
enables support for self-hosted servers in combination with the iOS app. This is to overcome the restrictive
Apple development environment.

Features:

• Regularly send Firebase keepalive messages to ~poll topic to support self-hosted servers (no ticket)
• Add subscribe filter to query exact messages by ID (no ticket)
• Support for poll_request messages to support iOS push notifications for self-hosted servers (no ticket)

Bugs:

• Support emails without Content-Type (#265, thanks to @dmbonsall)

Additional translations:

• Italian (thanks to @Genio2003)