Use avatar component

[rullzer]

It seems the old jquery method is still used here. Moving the avatar to the vuecomponents would be nice.

CC: @juliushaertl @skjnldsv

[nickvergessen]

Avatars are only used in the rich object strings: https://github.com/nextcloud/notifications/blob/master/src/richObjectStringParser.js

I could not make this dynamic templating work with Vue, so not sure this is possible.