MM-10417 Add local image proxy and enable by default (#9967)

* MM-10417 Add local image proxy and enable by default

* Remove unused function

* Add dependencies for willnorris/imageproxy

* Fixed compilation errors

* Lock to the master version of willnorris/imageproxy

* Fix atmos/camo proxy when no SiteURL is specified

* Re-add default values for deprecated settings

* Fix unit tests added by merge

* Pass imageproxy to App struct

* Remove unneeded locking when creating the image proxy

* Remove empty test file

vendor/github.com/gregjones/httpcache/README.md

@@ -0,0 +1,25 @@
+httpcache
+=========
+
+[![Build Status](https://travis-ci.org/gregjones/httpcache.svg?branch=master)](https://travis-ci.org/gregjones/httpcache) [![GoDoc](https://godoc.org/github.com/gregjones/httpcache?status.svg)](https://godoc.org/github.com/gregjones/httpcache)
+
+Package httpcache provides a http.RoundTripper implementation that works as a mostly [RFC 7234](https://tools.ietf.org/html/rfc7234) compliant cache for http responses.
+
+It is only suitable for use as a 'private' cache (i.e. for a web-browser or an API-client and not for a shared proxy).
+
+Cache Backends
+--------------
+
+- The built-in 'memory' cache stores responses in an in-memory map.
+- [`github.com/gregjones/httpcache/diskcache`](https://github.com/gregjones/httpcache/tree/master/diskcache) provides a filesystem-backed cache using the [diskv](https://github.com/peterbourgon/diskv) library.
+- [`github.com/gregjones/httpcache/memcache`](https://github.com/gregjones/httpcache/tree/master/memcache) provides memcache implementations, for both App Engine and 'normal' memcache servers.
+- [`sourcegraph.com/sourcegraph/s3cache`](https://sourcegraph.com/github.com/sourcegraph/s3cache) uses Amazon S3 for storage.
+- [`github.com/gregjones/httpcache/leveldbcache`](https://github.com/gregjones/httpcache/tree/master/leveldbcache) provides a filesystem-backed cache using [leveldb](https://github.com/syndtr/goleveldb/leveldb).
+- [`github.com/die-net/lrucache`](https://github.com/die-net/lrucache) provides an in-memory cache that will evict least-recently used entries.
+- [`github.com/die-net/lrucache/twotier`](https://github.com/die-net/lrucache/tree/master/twotier) allows caches to be combined, for example to use lrucache above with a persistent disk-cache.
+- [`github.com/birkelund/boltdbcache`](https://github.com/birkelund/boltdbcache) provides a BoltDB implementation (based on the [bbolt](https://github.com/coreos/bbolt) fork).
+
+License
+-------
+
+-	[MIT License](LICENSE.txt)