Add target="_blank" to links that don't have it
This is now more of a problem in the Electron app because of
https://github.com/electron/electron/issues/8841 but is still
annoying in the webapp if you're taken away from your chat client.
Exception is the SSO link, as commented (issue filed at
https://github.com/vector-im/riot-web/issues/8247).
Fixes https://github.com/vector-im/riot-web/issues/8226
