IPFS gateway settings

This change adds some settings for adding/configuring IPFS gateways. The settings are store in Preferences.

Here are some screenshots:

here are my TODOs before this branch is ready:

• figure out how to store settings -> shared preferences
• implement UI <- implemented with jet-pack compose and works very well for me. (Though, my experience with compose is limited and there might be some compose specific obscurities, I'm not aware about that would improve the code.)
• update downloader to use IPFS gateways from the new preferences
• display the number of active gateways (or disabled) in preference summary text
• move IPFS settings to expert settings
• double check all changed strings

closes #2541 (closed)
closes #2668 (closed)