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)

[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

See analysis details on SonarQube Cloud