Render descriptions with shared Markdown preview

Use one Markdown presentation for free-form content.

- issue and pull-request descriptions and comments
- milestone and commit descriptions

ios/Sources/ContentScreens.swift

@@ -733,7 +733,8 @@ final class MilestoneViewController: RefreshingTableViewController {
 final class MilestoneCell: UITableViewCell {
     private let stateIcon = UIImageView()
     private let titleLabel = UILabel()
-    private let descriptionLabel = UILabel()
+    private let stack = UIStackView()
+    private var descriptionView: UIView?
     private let metaLabel = UILabel()
     private let progress = UIProgressView(progressViewStyle: .bar)
 
@@ -744,16 +745,13 @@ final class MilestoneCell: UITableViewCell {
         let titleStack = UIStackView(arrangedSubviews: [stateIcon, titleLabel])
         titleStack.alignment = .firstBaseline
         titleStack.spacing = 8
-        descriptionLabel.font = .preferredFont(forTextStyle: .subheadline)
-        descriptionLabel.textColor = .secondaryLabel
-        descriptionLabel.numberOfLines = 2
         metaLabel.font = .preferredFont(forTextStyle: .caption1)
         metaLabel.textColor = .tertiaryLabel
         metaLabel.numberOfLines = 2
         progress.progressTintColor = .systemGreen
-        let stack = UIStackView(
-            arrangedSubviews: [titleStack, descriptionLabel, progress, metaLabel]
-        )
+        stack.addArrangedSubview(titleStack)
+        stack.addArrangedSubview(progress)
+        stack.addArrangedSubview(metaLabel)
         stack.axis = .vertical
         stack.spacing = 7
         stack.translatesAutoresizingMaskIntoConstraints = false
@@ -778,7 +776,12 @@ final class MilestoneCell: UITableViewCell {
             textStyle: .headline
         )
         titleLabel.text = row.title
-        descriptionLabel.text = row.description
+        descriptionView?.removeFromSuperview()
+        if !row.description.isEmpty {
+            let view = markdownView(row.description)
+            stack.insertArrangedSubview(view, at: 1)
+            descriptionView = view
+        }
         metaLabel.text = row.meta
         progress.progress = Float(row.progress)
         progress.trackTintColor = row.hasIssues ? .systemOrange : .systemGray5