Extend social meta tags

[AnnMarieW]

closes #2699

Currently, social media meta tags are automatically added for apps using Dash Pages.

This PR extends the existing include_pages_meta to regular Dash apps. The meta tags will be added if there is an app.descrption or app.image.

The app.image can be an asset filename or a URL.

For Pages apps, the app-level values provide defaults that can be overridden by page-level metadata.

Question:

• Should there be a default image? If so, what's the best way to add one - using a URL?
• If a default image is added, then the metatags would be generated for all apps. Should it be opt-in?

To do:

• add tests
• add changelog
• write docs and forum post

sample app

from dash import Dash, html

app = Dash(title="my title", description="my description", image="my_app.png")

app.layout = html.Div(["Hey!  Meta tags!"])

if __name__ == "__main__":
    app.run(debug=True)

[camdecoster]

Thanks Ann Marie! Philippe will take a look soon.

[T4rk1n]

Should there be a default image? If so, what's the best way to add one - using a URL?

I don't think we should add a default image, the image should be specific to the app to make it stand out.

We also have thumbnails and card on plotly cloud that are auto generated from the content, I think those are better than a generic default.

Code looks good, just need to add some tests.

[sonarqubecloud]

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[T4rk1n]

💃

[AnnMarieW]

@T4rk1n

I forgot to add a changelog. Would you like a separate PR?

Also, this PR makes the image for pages accept either a path to a file or a url. This makes the image_url unnecessary. Should it be deprecated? Or just the docstrings for image and image_url updated?

• image_url:
  Overrides the image property and sets the <image> meta tag to the provided image URL.

https://github.com/plotly/dash/blob/dev/dash/_pages.py#L241