[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/code_span.go

@@ -0,0 +1,83 @@
+package parser
+
+import (
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+type codeSpanParser struct {
+}
+
+var defaultCodeSpanParser = &codeSpanParser{}
+
+// NewCodeSpanParser return a new InlineParser that parses inline codes
+// surrounded by '`' .
+func NewCodeSpanParser() InlineParser {
+	return defaultCodeSpanParser
+}
+
+func (s *codeSpanParser) Trigger() []byte {
+	return []byte{'`'}
+}
+
+func (s *codeSpanParser) Parse(parent ast.Node, block text.Reader, pc Context) ast.Node {
+	line, startSegment := block.PeekLine()
+	opener := 0
+	for ; opener < len(line) && line[opener] == '`'; opener++ {
+	}
+	block.Advance(opener)
+	l, pos := block.Position()
+	node := ast.NewCodeSpan()
+	for {
+		line, segment := block.PeekLine()
+		if line == nil {
+			block.SetPosition(l, pos)
+			return ast.NewTextSegment(startSegment.WithStop(startSegment.Start + opener))
+		}
+		for i := 0; i < len(line); i++ {
+			c := line[i]
+			if c == '`' {
+				oldi := i
+				for ; i < len(line) && line[i] == '`'; i++ {
+				}
+				closure := i - oldi
+				if closure == opener && (i >= len(line) || line[i] != '`') {
+					segment = segment.WithStop(segment.Start + i - closure)
+					if !segment.IsEmpty() {
+						node.AppendChild(node, ast.NewRawTextSegment(segment))
+					}
+					block.Advance(i)
+					goto end
+				}
+			}
+		}
+		if !util.IsBlank(line) {
+			node.AppendChild(node, ast.NewRawTextSegment(segment))
+		}
+		block.AdvanceLine()
+	}
+end:
+	if !node.IsBlank(block.Source()) {
+		// trim first halfspace and last halfspace
+		segment := node.FirstChild().(*ast.Text).Segment
+		shouldTrimmed := true
+		if !(!segment.IsEmpty() && block.Source()[segment.Start] == ' ') {
+			shouldTrimmed = false
+		}
+		segment = node.LastChild().(*ast.Text).Segment
+		if !(!segment.IsEmpty() && block.Source()[segment.Stop-1] == ' ') {
+			shouldTrimmed = false
+		}
+		if shouldTrimmed {
+			t := node.FirstChild().(*ast.Text)
+			segment := t.Segment
+			t.Segment = segment.WithStart(segment.Start + 1)
+			t = node.LastChild().(*ast.Text)
+			segment = node.LastChild().(*ast.Text).Segment
+			t.Segment = segment.WithStop(segment.Stop - 1)
+		}
+
+	}
+	return node
+}