Bedrock: the page-index page has both page-index and index active

[Wolfr]

As reported in https://github.com/smartcoop/design/issues/36

In the main menu (the one accessible with the ctrl-m shortcut), the active page is highlighted. If the page is page-index, it highlights both page-index and index. See screenshot:

Can we make the selection on the page state a bit more strict?

[Wolfr]

I fixed it, and interestingly I also fixed a 5-year old bug by checking this code: https://github.com/usebedrock/bedrock/issues/178

However I still don't think it is advisable to use index.html for page template names. For the server,/folder/ is the same as /folder/index.html. Needs more testing.