Release v2.3.0

MRs:

• Improve gitaly info refs error message !172 (merged)
• Migrate GetBlob to Gitaly !174 (merged)
• Drop support for Go <1.8 !176 (merged)
• Add some tests for gzipped assets !177 (merged)
• Use reader/writer from gitaly streamio !178 (merged)
• Use http.Request contexts for Gitaly calls !179 (merged)
• Allow to access remote archive !180 (merged)