CWRC-Writer makes RDF xml invalid upon saving

When saving an XML document that contains RDF annotations, the function [buildAnnotations](https://github.com/cwrc/CWRC-Writer/blob/53d9227116e2428aadedc502be5051def17d8cdb/src/js/converter.js#L187-L277) is called to reinsert the RDF representing the annotations. 

This function builds RDF XML namespace attributes like so:

```
<rdf:RDF xmlns:rdf="xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"" xmlns:cw="xmlns:cw="http://cwrc.ca/ns/cw#"">
```

Note the repeating prefix.

This is due to using the namespace prefix in the [default namespaces](https://github.com/cwrc/CWRC-Writer/blob/53d9227116e2428aadedc502be5051def17d8cdb/src/js/converter.js#L191-L192) and then adding them again when building the [RDF string](https://github.com/cwrc/CWRC-Writer/blob/53d9227116e2428aadedc502be5051def17d8cdb/src/js/converter.js#L270-L272).