Navigation between assets using browser buttons

[Szab]

This pull requests adds the ability to navigate through the M365 learning pathways viewer webpart using the browser's navigation buttons (Back and Forward buttons).

It adds two new properties to the property pane:

1. Change browser URL when navigating between assets - toggle, this property is off by default. When disabled, the behaviour of M365 Learning Pathways doesn't change. When enabled, navigating between categories, playlists and assets will change the URL in the browser and make the Back and Forward buttons functional.
2. URL of the default viewer page - text, this property is empty by default. As of now, when generating links to pages, it is assumed that the viewer page is located on the page under the URL of {M365LPPortal}/SitePages/CustomLearningViewer.aspx. This property allows administrators to provide a different URL, effectively enabling them to rename the page without breaking the portal.

Providing the ability to both rename the viewer page (e.g. to simpler names such as Learning.aspx) and to navigate through the portal using the browser's navigation buttons greatly improves the end-user experience, eliminating the problem of navigating off the page by mistake.

[Szab]

@juliemturner @dcashpeterson Hi! When you have a moment, could you please look at the changes I proposed? I think they greatly improve the experience of navigating between the articles in the Learning Pathways.

[dcashpeterson]

@Szab I will look at this when I have time to process PRs. Thank you for the submission.

[Szab]

@dcashpeterson Sorry for pinging but can I ask for a review sometime?

[dcashpeterson]

@Szab sorry for the delay on this. We are actually in process of updating the HTML and CSS on the webpart to make it more accessible. I will let the development team know about this PR.