Force selection of annotation namespace

[MarcusBarnes]

What does this Pull Request do?

Removes the default annotation namespace setting and makes filling this configuration setting required. See issue #220.

What's new?

Does not pre-fill the annotation namespace configuration setting and also makes this setting required. Updates the form language for this setting too.

How should this be tested?

• If a prior version of the Web annotations module is installed, uninstall the module.
• Pull the changes and reinstall the module. If the Annotation Namespace setting is prefilled, uninstall the module again as this update clears the annotation namespace variable from the variables table on uninstall.
• Reinstall (enable) the module.
• In the Web Annotations module configuration, the Annotation Namespace textfield should be empty.
• Attempt to save the form without filling out this textfied. You should see a message saying that the Annotation Namespace is required.
• Fill out the Annotation Namespace textfield with your desired annotation namespace. You should be able to save the configuration.
• Create a new annotation - the new annotation object should use the namespace used in the Annotation Namespace configuration setting.

Additional Notes:

@Natkeeran It would be good to check that we remove other Web Annotation variables that are saved to Drupal's variables table on module uninstall.

[MarcusBarnes]

Need to pull in the changes made in pull-request https://github.com/digitalutsc/islandora_web_annotations/pull/227 so that this pull-request will pass the Travis CI build. I'll close and re-open.