PLT-3383: image proxy support (#7991)

* image proxy support

* go vet fix, remove mistakenly added coverage file

* fix test compile error

* add validation to config settings and documentation to model functions

* add message_source field to post

utils/markdown/lines.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package markdown
+
+type Line struct {
+	Range
+}
+
+func ParseLines(markdown string) (lines []Line) {
+	lineStartPosition := 0
+	isAfterCarriageReturn := false
+	for position, r := range markdown {
+		if r == '\n' {
+			lines = append(lines, Line{Range{lineStartPosition, position + 1}})
+			lineStartPosition = position + 1
+		} else if isAfterCarriageReturn {
+			lines = append(lines, Line{Range{lineStartPosition, position}})
+			lineStartPosition = position
+		}
+		isAfterCarriageReturn = r == '\r'
+	}
+	if lineStartPosition < len(markdown) {
+		lines = append(lines, Line{Range{lineStartPosition, len(markdown)}})
+	}
+	return
+}