IM: Terms modal: Make computation of baseUrl more generic

Riot/Managers/Widgets/WidgetManagerConfig.swift

@@ -36,6 +36,38 @@ class WidgetManagerConfig: NSObject, NSCoding {
         }
     }
 
+    var baseUrl: NSString? {
+        // Same comment as https://github.com/matrix-org/matrix-react-sdk/blob/1b0d8510a2ee93beddcd34c2d5770aa9fc76b1d9/src/ScalarAuthClient.js#L108
+        // The terms endpoints are new and so live on standard _matrix prefixes,
+        // but IM rest urls are currently configured with paths, so remove the
+        // path from the base URL before passing it to the js-sdk
+
+        // We continue to use the full URL for the calls done by
+        // Riot-iOS, but the standard terms API called
+        // by the matrix-ios-sdk lives on the standard _matrix path. This means we
+        // don't support running IMs on a non-root path, but it's the only
+        // realistic way of transitioning to _matrix paths since configs in
+        // the wild contain bits of the API path.
+
+        // Once we've fully transitioned to _matrix URLs, we can give people
+        // a grace period to update their configs, then use the rest url as
+        // a regular base url.
+        guard let apiUrl = self.apiUrl as String?, let imApiUrl = URL(string: apiUrl) else {
+            return nil
+        }
+
+        guard var baseUrl = URL(string: "/", relativeTo: imApiUrl)?.absoluteString else {
+            return nil
+        }
+
+        if baseUrl.hasSuffix("/") {
+            // SDK doest not like trailing /
+            baseUrl = String(baseUrl.dropLast())
+        }
+
+        return baseUrl as NSString
+    }
+
     init(apiUrl: NSString?, uiUrl: NSString?) {
         self.apiUrl = apiUrl
         self.uiUrl = uiUrl