URL previewing support

[ara4n]

• Splits thumbnail height calculation for images into a new ImageUtils class so it can be used by both image event thumbnailing and URL preview thumbnailing
• Adds a LinkPreviewWidget component for rendering URL previews, used within TextualBody
• Update TextualBody to look for the links within its body, and create a URL preview of the first link it finds.
• Support hiding and unhiding previews via mxEvent.widgetHidden (and persisted in localStorage as a short term soln)
• Kill off the old onImageLoad callbacks on ScrollPanel and friends, and immediately resurrect them as onWidgetLoad callbacks to let ScrollPanel maintain its scroll offset correctly whilst URL previews are being dynamically loaded.

This is part of a set of PRs spanning vector-web, matrix-react-sdk, matrix-js-sdk and synapse.
See also element-hq/element-web#1343

[richvdh]

Now that this is a public function, it would be nice to give this a comment saying what it does and what the parameters mean. How does its result relate to the thumbHeight param? Which of the params are allowed to be undefined? I could figure it out, but I shouldn't have to.

[richvdh]

done

[ara4n]

@richvdh PTAL - a few bits aren't resolved yet but are pending more feedback.

[richvdh]

So I think remaining things here are:

• rewire the hiddenness flag/unhiding
• forceupdate in MessagePanel._onWidgetLoad

[ara4n]

@richvdh done; feels much nicer to me. PTAL

[richvdh]

you don't need to do checkScroll as well as forceUpdate.

[ara4n]

@richvdh PTAL again?

[richvdh]

lgtm