Upgrade to Grails 2.3.0

I upgrade the application to Grails 2.3.0 and change some things that
need manual attention.
Upgrade many plugins

I upgrade a lot of plugins to their newest version.
Upgrade resource plugins

I upgrade the resource plugin and the compiler plugins for LESS and
CoffeeScript to their newest versions.
Licensing issues

First, I rework the license section on the about page.  I remove license
hints which are no longer needed and add new license descriptions.
Also, I link the GPL, SIL, and MIT licenses.

Furthermore, I simplify the logo styles on the login page.
Plugin upgrades/bug fixes

First, I upgrade some plugins to their newest versions.

Then, I fix a bug in DataSource.groovy that a DataSource property
doesn't exist any longer in Tomcat 7.  Also I fix a NPE in
InstallService when accessing ServletContext.
Modifications due to functional tests

I run all functional tests with the new Grails 2.3.1 upgrade and fix a
lot of things:

- Use new ValueConverter classes instead of the old PropertyEditor
  classes due to changes in Grails 2.3
- Output raw values in ${...} blocks because they are now HTML encoded
  by default
- Rework the test cases that test the note functionality by removing the
  old TinyMCE code
- Fix validation errors in sales item classes
- Add additional constraint for unit in InvoicingItem

Furthermore, I restructure the Config.groovy for better overview.
Merge branch 'sandbox/grails-2.3' into dev

Conflicts:
	application.properties