Add default push rule to ignore reactions
This adds a default push rule to ignore reactions as proposed in
[MSC2153](https://github.com/matrix-org/matrix-doc/pull/2153). By adding it here
in the client directly, we can try out the idea early even if it hasn't appeared
in the user's HS yet.
Part of https://github.com/vector-im/riot-web/issues/10208
