Pretty URLs break “Edit this page” links

## Expected Behavior

While reporting https://github.com/dita-ot/docs/issues/607, @stefan-jung discovered:

> I tried to modify the following topic with the **Edit this page** button:
> https://www.dita-ot.org/dev/topics/plugin-messages#ariaid-title1

— but it didn't work.

## Actual Behavior

[oXygen XML Web Author](https://www.oxygenxml.com/xml_web_author.html) returns an error that it can't find the resource to edit.

Looks like a side-effect of Netlify’s post-processing for [pretty URLs](https://docs.netlify.com/site-deploys/post-processing/#post-processing-features).

At first glance, it looked like the trailing fragment identifier `#ariaid-title1` that Algolia adds when you follow links in the site search results could be to blame.

But stripping that isn’t enough. Web Author also doesn’t find the topic under [topics/plugin-messages](https://www.dita-ot.org/dev/topics/plugin-messages).

It seems Web Author relies on the `.html` file name extension to locate the corresponding `.dita` source file.

If you manually edit the URL in your browser to [topics/plugin-messages.html](https://www.dita-ot.org/dev/topics/plugin-messages.html) and then click **Edit this page**, it works.

## Possible Solution

We’d probably need to edit the logic in the `parse()` function on [lines 55–56 of the EditController.js](https://github.com/dita-ot/website/blob/master/_js/lib/EditController.js#L55-L56) file.

Not sure if there's a concise (and resilient) way of refactoring that that would work with the pretty URL syntax. 🤔

One thing that comes to mind would be walking the URL backward from the end to find the last slash `/`, then using the final sequence of letters, dashes, or underscores that construct the root filename, ignoring any trailing fragment identifiers.

— but there may be a better way.

@raducoravu, @jelovirt Any suggestions?