Clean up donate URL code

This small change cleans up two things for the donate URL:

1. Moves it into a constant in `TSConstants`
2. Adds a trailing slash to the URL (because the server will redirect
   you without one)

I think these cleanups are useful by themselves, but I'll be using the
constant again in an upcoming change and split this off into its own
small commit.

Made sure I could still open the URL in the app.

Signal/src/ViewControllers/AppSettings/AppSettingsViewController.swift

@@ -318,7 +318,7 @@ class AppSettingsViewController: OWSTableViewController2 {
                                accessoryView: nil,
                                accessibilityIdentifier: UIView.accessibilityIdentifier(in: self, name: "donate"),
                                actionBlock: { () in
-                UIApplication.shared.open(URL(string: "https://signal.org/donate")!, options: [:], completionHandler: nil)
+                UIApplication.shared.open(TSConstants.donateUrl, options: [:], completionHandler: nil)
             }))
         }
 

Signal/src/ViewControllers/AppSettings/BoostViewController.swift

@@ -336,7 +336,7 @@ class BoostViewController: OWSTableViewController2 {
     }
 
     private func openDonateWebsite() {
-        UIApplication.shared.open(URL(string: "https://signal.org/donate")!, options: [:], completionHandler: nil)
+        UIApplication.shared.open(TSConstants.donateUrl, options: [:], completionHandler: nil)
     }
 }
 

SignalServiceKit/src/TSConstants.swift

@@ -21,6 +21,7 @@ public class TSConstants: NSObject {
     private override init() {}
 
     public static let legalTermsUrl = URL(string: "https://signal.org/legal/")!
+    public static let donateUrl = URL(string: "https://signal.org/donate/")!
 
     @objc
     public static var mainServiceWebSocketAPI_identified: String { shared.mainServiceWebSocketAPI_identified }