New features:
* Attachments: Send files to the phone (#25, #15)
* Click action: Add a click action URL to notifications (#85)
* Battery optimization: Allow disabling persistent wake-lock (#76, thanks @MatMaul)
* Recognize imported user CA certificate for self-hosted servers (#87, thanks @keith24)
* Remove mentions of "instant delivery" from F-Droid to make it less confusing (no ticket)

Bug fixes:
* Subscription "muted until" was not always respected (#90)
* Fix two stack traces reported by Play console vitals (no ticket)
* Truncate FCM messages >4,000 bytes, prefer instant messages (#84)
