Fixed EZP-21219: versionread policy not checked in ContentService::loadContent

JIRA issue: https://jira.ez.no/browse/EZP-21219

# Description

`ContentService::loadContent` allows to read any version of a content provided the user can read this content. This patch adds a check on content/versionread policy when a specific version is requested.

# Tests

manual tests + relevant unit test (waiting for travis to run the whole test suite)