Verify Element X with an existing Element Classic account. (#5374)

* Read and import the secrets from ClassicAppAccounts.

* Record snapshots.

* Add some documentation, tidy up tests and fix the dismissal of the backup instructions.

* Workaround flakey tests (the fulfilments weren't always firing).

* Allow a custom Classic App deep link URL to be configured.

ElementX/Sources/Other/InfoPlistReader.swift

@@ -27,6 +27,7 @@ struct InfoPlistReader {
         static let classicAppGroupIdentifier = "classicAppGroupIdentifier"
         static let classicAppKeychainServiceIdentifier = "classicAppKeychainServiceIdentifier"
         static let classicAppKeychainAccessGroupIdentifier = "classicAppKeychainAccessGroupIdentifier"
+        static let classicAppDeepLinkURL = "classicAppDeepLinkURL"
     }
     
     private enum Values {
@@ -133,6 +134,11 @@ struct InfoPlistReader {
         infoPlistValue(forKey: Keys.classicAppKeychainAccessGroupIdentifier)
     }
     
+    var classicAppDeepLinkURL: URL? {
+        let urlString: String? = infoPlistValue(forKey: Keys.classicAppDeepLinkURL)
+        return urlString.flatMap { URL(string: $0) }
+    }
+    
     // MARK: - Private
     
     @_disfavoredOverload // Make sure optional types default to the optional version below.