WMS - Implemented buffer/ratio configuration

The ratio (size mulutiplier for single-tile mode) and buffer (number of
extra rows/columns in tiled mode) for WMS service instances can now be
managed in the backend.
Merge branch 'feature/wms-buffer-ratio' into release/3.0.4.0

Conflicts:
	src/Mapbender/WmsBundle/Form/Type/WmsInstanceInstanceLayersType.php
	src/Mapbender/WmsBundle/Resources/public/mapbender.source.wms.js
Fix translation files