Night mode (dark theme)

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• Implemented dark mode switch
• Added saving actual mode
• Changed png icons to vector images

Webapp MR for the chat view, https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1232

gitter_new_sample

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git remote add charafau git@gitlab.com:charafau/gitter-android-app.git
git push charafau charafau/gitter-android-app-feat/dark_mode:feat/dark_mode

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Closes https://gitlab.com/gitlab-org/gitter/gitter-android-app/issues/58