Add pages for multi-page app

[AnnMarieW]

This PR moves the pages plug-in from dash-labs to dash.
Until the tests are written, the app in the usage folder can be used to try the new features.

To Do

• Find a better way to build dash.register_page. See dash-labs #76. Status: Deferred for now, see comments in dash-labs issue.
• How to call pages function only when those features are being used.
• Delete the usage folder
• Add entry in the CHANGELOG.md

Test coverage

• test navigation
  • by url
  • by dcc.Link
• test for path variables.
• test for query string
• test for adding arbitrary page information to dash.page_registry
• test for redirects
• test for the title
• test for the extra metadata different by page.
• test for layout as function.
• test invalid cases that should be handled.
  • no layout in page.
  • fail to validate the layout page container.
• test load pages from directory.
• test not found 404 page.
• test for specifying page order

Find a live demo of the pages features at https://dashlabs.pythonanywhere.com/

[AnnMarieW]

@alexcjohnson

changelog
How much information would you like in the CHANGELOG.md? Should it just be a pointer to the forum post?

pages function
What do you think about the following solution for handling when to call the pages function:
I added the following Dash args:

pages: Default False. Set to True to enable the multi-page apps features
pages_folder Default "pages". A path relative to the current working directory for files used with the multi-page app pages feature.

If a pages_folder name is supplied, then pages is set to True so it's not necessary to set both props.
In the unlikely event that people use the multi-page features without a pages/ folder then the warning is not shown if pages=True and pages_folder=None

See commit 4bdf472

[AnnMarieW]

This commit is a fix for the bug reported in dash-labs plotly/dash-labs#85

[T4rk1n]

This is a very nice feature, I like it.

I think now that it's in dash directly instead of dash-labs, we can have better integration for the index rendering.

I am also wondering if there would be a way to do without a dcc.Location and container to serve the proper page layout directly in _dash-layout instead of an additional callback.

[AnnMarieW]

@T4rk1n Thanks so much for your review!

Can you say more about how we might do this:

I am also wondering if there would be a way to do without a dcc.Location and container to serve the proper page layout directly in _dash-layout instead of an additional callback.

[T4rk1n]

In Dash.serve_layout, you can use flask.request.referrer to find the path of the page that sent the request. Could also wrap the layout in the container there so you still have the dcc.Store(id='_page_title', value=title) to update the title with the clientside callback.

[alexcjohnson]

💃 Love it! This has been an epic effort @AnnMarieW, can't wait to get it into everyone's hands!