BAC-215: ImageServing issues with galleries

Solves aforementioned bug and fb#98435 - images in galleries prefixed with ``Image:`` or ``File:`` were not indexed by ImageServing.

This pull request includes a small code cleanup:

* ImageServing API modules are now in ImageServing directory
* ``<image>`` placeholders generation moved to a helper method
* ``imageServing.php`` maintenance script added to test parsing and indexing of a given article
* more documentation