Allow to set feedback URL via environment variable

With this PR:
- the default Insight build should be unmodified and produce Insight artifacts which connect to http://qa.openmicroscopy.org.uk/ when sending feedback
- it is now possible to configure the base url of the QA feedback by setting the `QA_BASEURL` environment variable in the build command, e.g.

  ```
QA_BASEURL=http://qa.staging.openmicroscopy.org/qa ./build.py -f components/insight/build.xml dist
```

--no-rebase