Experimental: Removed lazyload.js and use postpone attribute instead

https://github.com/marienfressinaud/FreshRSS/issues/316
The performance of lazyload.js was not good enough, and not really
needed anyway.
This change mostly affects mainly situations when the content of
articles is shown by default, not so much when they are collapsed
Using HTML5 lazyload and postpone attributes by default on all img,
audio, iframe, video.
http://www.w3.org/TR/resource-priorities/#attr-postpone
Postpone attribute is removed by JavaScript if the user does not want
the lazyload behaviour.
In the case when users do want the lazyload behaviour, in normal view
with articles hidden, we furthermore use the data-original approach to
be sure to support current browsers.
+Corrected some bugs with enclosures, and some images not appearing
before the first scroll.
+Now faster regex processing img and iframe at once (was not practical
with lazyload.js)