refactor: Migrate subscription products cell to code

Sources/Earthquake Network/Controllers/InApp/SubscriptionsHeaderTableViewCell.swift

@@ -8,26 +8,53 @@
 
 import UIKit
 
-class SubscriptionsHeaderTableViewCell: UITableViewCell {
 
-    var isLoading = false {
-        didSet {
-            updateUI()
-        }
+class SubscriptionsHeaderTableViewCell: UITableViewHeaderFooterView {
+    
+    // MARK: - UI
+    
+    private lazy var headerTitleLabel: UILabel = {
+        let label = UILabel()
+        label.translatesAutoresizingMaskIntoConstraints = false
+        label.font = UIFont.preferredFont(forTextStyle: .title2)
+        label.textColor = AppTheme.Colors.darkGray
+        return label
+    }()
+    
+    private lazy var loadingActivityIndicator: UIActivityIndicatorView = {
+        let spinner = UIActivityIndicatorView(style: .medium)
+        spinner.translatesAutoresizingMaskIntoConstraints = false
+        spinner.hidesWhenStopped = true
+        return spinner
+    }()
+
+    // MARK: - Init
+    
+    override init(reuseIdentifier: String?) {
+        super.init(reuseIdentifier: reuseIdentifier)
+        setupUI()
     }
     
-    var title: String? = nil {
-        didSet {
-            updateUI()
-        }
+    required init?(coder: NSCoder) {
+        super.init(coder: coder)
+        setupUI()
     }
     
-    @IBOutlet private weak var headerTitleLabel: UILabel!
-    @IBOutlet private weak var loadingActivityIndicator: UIActivityIndicatorView!
-    
     // MARK: - Private
     
-    private func updateUI() {
+    private func setupUI() {
+        contentView.addSubview(headerTitleLabel)
+        contentView.addSubview(loadingActivityIndicator)
+        
+        headerTitleLabel.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
+        headerTitleLabel.leadingAnchor.constraint(equalTo: contentView.leadingAnchor, constant: .cardPadding).isActive = true
+        loadingActivityIndicator.trailingAnchor.constraint(equalTo: contentView.trailingAnchor, constant: .cardPadding.negative).isActive = true
+        loadingActivityIndicator.centerYAnchor.constraint(equalTo: headerTitleLabel.centerYAnchor).isActive = true
+    }
+    
+    // MARK: - Public
+    
+    func update(isLoading: Bool, title: String?) {
         headerTitleLabel.text = title
         
         if isLoading && title != nil {