Update HTML editor to support internal links better

[pjmonks]

Resolves #417

Updates to the HTML editor (Jodit) to support internal catalogue element links better:

1. Re-styled the toolbar button to look more like a link:

2. All links produced used to include the exact unique identifiers of the catalogue elements. This has been changed to match how the equivalent function in the Markdown editor works, where URLs are rendered based on named links, e.g.

http://localhost:4200/#/catalogue/codeSet/cs%3AComplex%20Test%20CodeSet

[jamesrwelch]

I think this solves some of the immediate problems. But we might have a larger piece of work in the near future to improve this... In particular the selection of things to link to.