Make insertContentType fix configurable in `td-builder`

[relu91]

In #52 we introduced a well-known fix for a problem in the Thing Description of the TDD (see w3c/wot-discovery#469). We should make it configurable from the ConfigService.

Tracking TODO:

zion/src/introduction/td-builder.service.ts

Line 34 in 2ad535d

this.insertResponseContentType();