Externalise remaining hardcoded messages

Currently, some messages are hardcoded and thus not translatable. They should be externalised to [src/i18n/en.json](https://github.com/knowsys/nemo-web/blob/main/src/i18n/en.json) and documented in [src/i18n/qqq.json](https://github.com/knowsys/nemo-web/blob/main/src/i18n/qqq.json).