Add titel, description and images for fastlane (and F-Droid) (#564)

* Add titel, description and move images for fastlane (and F-Droid)

* Replace app title with multi-account feature in the short description

* Add feature list to full_description.txt

fastlane/metadata/android/en-US/full_description.txt

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+Tusky is a client for Mastodon, a free and open-source social network server. Share words, photos, and videos with folks!
+
+• Material Design
+• Most Mastodon APIs implemented
+• Multi-Account support
+• Dark and light theme with the possibility to auto-switch based on the time of day
+• Drafts – compose toots and save them for later
+• Completely open-source – no non-free dependencies like Google services
+
+Mastodon is decentralized, meaning anyone can run Mastodon and participate in the social network seamlessly.
+
+To learn more about Mastodon, visit https://joinmastodon.org/
+
+Tusky is also itself open-source, it's source code is available at https://github.com/Vavassor/Tusky

fastlane/metadata/android/en-US/short_description.txt

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+A multi account client for the social network Mastodon

fastlane/metadata/android/en-US/title.txt

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+Tusky