Synchronize feature list in README and metadata

[PanderMusubi]

Please synchronize the feature list etc. between https://github.com/forkgram/TelegramAndroid/blob/dev/README.md and https://github.com/forkgram/TelegramAndroid/blob/dev/metadata/en-US/full_description.txt

Also mention UnifiedPush in both. Fixing this will be reflected in https://f-droid.org/packages/org.forkgram.messenger/

Additionally, add a feature image according to https://gitlab.com/-/snippets/1895688#image-sizes to metadata for fastlane. Currently, the F-Droid client app falls back to using the icon as feature image, which is shown at a low resolution because of this.