Add ability to autosave photos to iOS Photos app for a chat

**Is your feature request related to a problem? Please describe.**
Not really a "problem", but it would be a nice UX improvement if we could choose to save all photos shared in a chat to the phone gallery automatically.

**Describe the solution you'd like**
A chat-specific setting to enable or disable autosave of all photos shared in that chat.

**Describe alternatives you've considered**
The current alternative is to manually save every picture we're interested in.

**Additional context**
Other chat apps often offer this feature.