VideoPress: insert VideoPress.com URLs instead of shortcodes.

When inserting videos in posts and pages using Jetpack's VideoPress module, each video is inserted using a `[videopress]` shortcode. That shortcode can include multiple parameters. We also support the `[wpvideo]` shortcode, with similar parameters.

However, if you ever deactivate Jetpack from your site, all the shortcodes stop working and you're left with posts without videos. Similarly, if you move away from WordPress and export your content without touching the shortcodes, you'll be left with meaningless shortcodes in your content.

To solve these issues, we could insert full VideoPress URLs instead, like so:

`https://videopress.com/v/knHSQ2fb/`

If that URL is inserted on a separate line within the content, a video can be automatically rendered in all services that support oEmbed. That includes WordPress itself. It would then preserve everyone's content even when they stop using Jetpack.

Related: #3655, #6150

The issue names no files or tests. Start by locating Jetpack's VideoPress insertion flow and its handling of the [videopress] and [wpvideo] parameters; done means inserting the VideoPress URL on its own line while preserving video rendering in WordPress and other oEmbed consumers.

Written by the indexing model from the issue text.