Normalise pre-signed URLs pasted into editors #368

[bwalkerl]

Requires MDL-73596 to be backported to make use of this feature.

Testing instructions:

1. Setup objectfs and enable pre-signing
2. Open a pluginfile in a new page, you should see a full pre-signed URL. With this patch it should have a #objectfs-origin= at the end.
3. Open an editor. This can be mod_label (Text and media area) or anything else.
4. Copy the URL and paste it into an editor. If this is an image, the image will be shown. If not, you will see the full URL. Hyperlinks can also be tested.
5. Click save.
6. Open the editor and confirm that you see a pluginfile link instead. For images, confirm this by viewing the source code in the editor.

This will only work if the link has character encoding for spaces, which should be the default for copying from URL in browser. Chromium will always have the encoded URL, while Firefox will decode the URL but have users copy the encoded URL unless it has been modified. In such edge cases where the URL has been modified (or copied from a different source) the copied value can contain spaces instead of the encoded value, which will break the current handling (it should be obvious in the editor this is broken saving, so I don't think this is the main concern).

Closes #368
Closes #744