Fix assets to work when hosted at subdirectory

[antonymilne]

Description

A Vizro dashboard might be accessible through 3 different sorts of route:

1. root level, e.g. http://www.example.com/
2. subdomain, e.g. http://subdomain.example.com/ (as with Heroku for example)
3. subdirectory, e.g. http://www.example.com/path/

Before this change: 1 and 2 already worked correctly but 3 did not.
Now: all should work correctly.

I've tested 1, will test 2 soon. @maxschulz-COL please can you test 3? I've bumped the version here to 0.1.6.dev2 so you can check you're using the right version when you deploy.

Important

We now have a "proper" way to alter assets_folder from its default rather than going through Vizro._user_assets_folder, since I've exposed **kwargs in Vizro().

Here's the rules for how we should write code so that paths are always correctly formed:

• always use dash.get_relative_path to link to pages with href (see _make_page_404_layout example link)
• always use dash.get_relative_path(f"/{STATIC_URL_PREFIX}/..") to refer to built-in assets in the static folder (see _make_page_404_layout example html.Img)
• always use dash.get_asset_url to refer to things in the user assets folder (we don't have any examples of this at the moment outside docs)

Still to do:

• test case 2
• test case 3 @maxschulz-COL
• update changelog
• update deployment docs
• update docs with the above rules
• update docs about how to specify assets folder and how we can now do **kwargs in Vizro.__init__

Checklist

• I have not referenced individuals, products or companies in any commits, directly or indirectly
• I have not added data or restricted code in any commits, directly or indirectly
• I have updated the docstring of any public function/class/model changed
• I have added tests to cover my changes (if applicable)

Types of changes

• Docs/refactoring (non-breaking change which improves codebase)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Notice

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[maxschulz-COL]

Great stuff! Couple of QQ

[maxschulz-COL]

LG from my side!

[huong-li-nguyen]

This looks great - thanks for fixing and solving the mystery! 🚀 🔥

Looks very clean now - I also like that you don't have to provide an absolute path to the assets_folder anymore. This has been confusing for some users in the past!