Support clear reactions in e2e rooms

[manuroe]

As explained by Ryan at #2448 (comment) for riot-web

[manuroe]

Kamino cloned this issue to vector-im/riotX-android

[manuroe]

It works in phase:1 but for main use cases. e2e will be officially supported in phase:2.

[jryans]

Riot Web has changed to special case reactions so that they are sent without encryption to avoid notifications.