Nextcloud SingleSignOn 0.8.0

• Enhance exception handling !558 (thanks @stefan-niedermann)
• Remove outdated icon from error dialog !556 (thanks @stefan-niedermann)
• Implement java.lang.AutoClosable !546 (thanks @stefan-niedermann)
  • Use close() instead of stop() on the Nextcloud API.
• Add consumer proguard rules !545 (thanks @Unpublished)
  • New docs available: https://github.com/nextcloud/Android-SingleSignOn#r8proguard
• Use stream with try-with-resources !544 (thanks @Unpublished)
• Add response headers to responses for Call<T> Retrofit methods !543 (thanks @desperateCoder)
• Dependency updates