Fixes a bug introduced in
https://github.com/matrix-org/matrix-react-sdk/pull/426.
Particularly when we are showing search results, we may not recognise the
sender of an event; attempting to create a MemberAvatar for it will lead to
null-reference errors.
Also a bit of untangling of the logic of needsSenderProfile. Since
https://github.com/matrix-org/matrix-react-sdk/pull/422,
EventTileType.needsSenderProfile was only being called on MessageEvents, and
therefore only returned true. It's a shame to see all this logic going into
EventTile rather than the individual EventTileTypes, but since it's there,
let's not leave the unused logic lying around in the EventTileType
implementations.
Switch to using a normal <a href="..."> link for search result
clickthrough. Apart from generally giving a better experience, this means that
it also works on html messages. The problem there was that we were attaching
onClick handlers to <span>s which we were then flattening into HTML with
ReactDOMServer (which meant the onClick handlers were never attached to React's
list of listeners).
To make this work without jumping through React hoops, the highlighter now
returns either a list of strings or a list of nodes, depending on whether we
are dealing with an HTML event or a text one. We therefore have a separate
HtmlHighlighter and TextHighlighter.
