Series import: validate app.site-parser[x].*-locator properties

[php-coder]

Let's use QueryParser.parse(locator) to ensue that properties are valid locators. If they aren't we will handle SelectorParseException and fail-fast during application initialization.

[php-coder]

After fixing #975 the properties will live in database and we won't have code that validates the properties (see also #842 ). I'm not sure that we need validation for them at this stage because such errors should not happen as only admin adds these properties to database. Even if they occurs, the chance is really small and in that case we may circle back to this discussion.