[MM-26812] Add support for resumable file uploads (#15252)

* Implement AppendFile for FileBackend

* Split test into subtests

* [MM-26812] Add support for resumable file uploads (#15252)

* Implement UploadSession

* Implement UploadSessionStore

* Add error strings

* Implement resumable file uploads

* Add UploadType

* Fix retry layer tests

* Regenerate store layers

* Fix store error handling

* Use base for filename

* Prevent concurrent uploads on the same upload session

* Fix erroneus error string

* Improve error handling

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

* Fix translations

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

go.tools.mod

@@ -4,7 +4,7 @@ go 1.14
 
 require (
 	github.com/jstemmer/go-junit-report v0.9.1 // indirect
-	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200828152206-519b99a4e51e // indirect
+	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200915114419-f4421bc07461 // indirect
 	github.com/philhofer/fwd v1.0.0 // indirect
 	github.com/reflog/struct2interface v0.6.1 // indirect
 	github.com/tinylib/msgp v1.1.2 // indirect