[MM-11143] Strip markdown formatting characters from push notifications (#17775)

* WIP: trim markdown for push notification

* fix: golangci

* fix: table regex

* fix: regex code block

* doc: add license

* WIP: custom renderer with goldmark

* WIP: update goldmark version to 1.38

* fix: use goldmark as parser

* fix: remove table extension

* fix: change buf to `strings.Builder`

* fix: return original string, log warning if error

* refactor: change `WriteString` to `WriteByte`

* refactor: change if condition

* refactor: use assertion

* refactor: move to inline

* fix: remove handle multiline

Already handled by mobile

* refactor: wrap same function

* refactor: move strip markdown to `sendPushNotificationSync`

* refactor: renaming variable aren't used

* fix: move log to func `sendPushNotificationSync`

* docs: add comment to `StripMarkdown`

* fix: move assign message to else

* appErr to err rename

Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

vendor/github.com/yuin/goldmark/parser/paragraph.go

@@ -0,0 +1,71 @@
+package parser
+
+import (
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+)
+
+type paragraphParser struct {
+}
+
+var defaultParagraphParser = &paragraphParser{}
+
+// NewParagraphParser returns a new BlockParser that
+// parses paragraphs.
+func NewParagraphParser() BlockParser {
+	return defaultParagraphParser
+}
+
+func (b *paragraphParser) Trigger() []byte {
+	return nil
+}
+
+func (b *paragraphParser) Open(parent ast.Node, reader text.Reader, pc Context) (ast.Node, State) {
+	_, segment := reader.PeekLine()
+	segment = segment.TrimLeftSpace(reader.Source())
+	if segment.IsEmpty() {
+		return nil, NoChildren
+	}
+	node := ast.NewParagraph()
+	node.Lines().Append(segment)
+	reader.Advance(segment.Len() - 1)
+	return node, NoChildren
+}
+
+func (b *paragraphParser) Continue(node ast.Node, reader text.Reader, pc Context) State {
+	_, segment := reader.PeekLine()
+	segment = segment.TrimLeftSpace(reader.Source())
+	if segment.IsEmpty() {
+		return Close
+	}
+	node.Lines().Append(segment)
+	reader.Advance(segment.Len() - 1)
+	return Continue | NoChildren
+}
+
+func (b *paragraphParser) Close(node ast.Node, reader text.Reader, pc Context) {
+	parent := node.Parent()
+	if parent == nil {
+		// paragraph has been transformed
+		return
+	}
+	lines := node.Lines()
+	if lines.Len() != 0 {
+		// trim trailing spaces
+		length := lines.Len()
+		lastLine := node.Lines().At(length - 1)
+		node.Lines().Set(length-1, lastLine.TrimRightSpace(reader.Source()))
+	}
+	if lines.Len() == 0 {
+		node.Parent().RemoveChild(node.Parent(), node)
+		return
+	}
+}
+
+func (b *paragraphParser) CanInterruptParagraph() bool {
+	return false
+}
+
+func (b *paragraphParser) CanAcceptIndentedLine() bool {
+	return false
+}