New plugin front-end helper function: navigateByUrl

[JohnXLivingston]

Describe the problem to be solved

If a plugin wants to navigate to another page (for example after having saved something), and uses <a> tag or something like window.location.href = '/p/xxxx', currently the page will reload (despite Peertube is a one-page app).

The correct way to navigate using Angular is something like router.navigateByUrl('/p/xxx'), but plugins don't have access to angular router and angular methods.

Describe the solution you would like

Front-end helpers should include a method to navigate to angular routes. Something like peertubeHelpers.navigateByUrl(url) which could be bind to the associated angular method.