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.
96567dad0d