Improve EPSG code for Mercator projection

[eblondel]

EPSG:3349 is used in FigisMap and figismapdata moniker. However this projection is deprecated see http://www.opengis.net/def/crs/EPSG/0/3349

EPSG:900913 is not a valid EPSG code http://www.opengis.net/def/crs/EPSG/0/900913

The correct Mercator projection should be: http://www.opengis.net/def/crs/EPSG/0/3857

Proposal: Evaluate with RFB data editor the possibility to align figismapdata and FigisMap.js on the approved EPSG code.

[eblondel]

Although EPSG:900913 is not valid EPSG code according to OGC CRS definitions. It will be kept as alias for EPSG:3857. The proj4 definition is exactly the same, but the EPSG code passed to FigisMap will be EPSG:900913, because the GWC has currently tiles generated with the projection. The html label will mention the official code (EPSG:3857)