Generate pills from display names

### Your use case

#### What would you like to do?
Pills are generated from valid `label` markup with a matrix.to link that points to a user or room.
On web, the labels of these pills are generated by element to show the current displayname of the target entity.
Sending a custom html message and viewing on android, it seems that the provided label is used.

Please decide to adopt web's behaviour.

#### Why would you like to do it?
parity

#### How would you like to achieve it?
/

### Have you considered any alternatives?

If not adopt, decide on another common behaviour. I'm not sure but maybe this should even be spec'd.

### Additional context

related #1046 seems to be obsolete