This commit adds the migration code that converts all Feeds Importers which used
the DateIcalIcalcreatorParser plugin to use DateiCalFeedsImporter instead. The
old classes have been removed.

Applied patch #11 from [#1922658] to get around iCalcreator limitation with stream wrappers. Also made the amount of whitespace on blank lines consistent throughout the project.

Includes patches from issues 1895396 and 1462646.
Updated the README and the CHANGELOG.