Mmmonk feedback: remove "__test__/integration" from demo URLs

[marlo-longley]

This report by Mmmonk is based on feedback from 300 users. One finding is that technical language throughout the app is making users uncomfortable.

The _test__/integration path for demo content was specifically identified as "scary."

As @lutzhelm writes up in this overview:

The Mirador demo on Netlify does not only provide the well known default demo whose source code can be found here. Besides their usage in automated integration tests, the HTML files in __tests__/integration/mirador are also served by Netlify and are thus publicly available.

Can we make these URLs friendlier?

Broken out from #3906