in-app image media preview

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refactor ListenableFutureTask and make saves async

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replace ABS with AppCompat

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enable proguard

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shorten blocks of text

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padding workaround for bug in appcompat-v7 21.0.0

bug: https://code.google.com/p/android/issues/detail?id=77982

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clear pending slides when attachment reselected

Fixes #2012

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