leaflet page should free from using proj4js

[boney-bun]

Problem

atm, leaflet pages use proj4js library to check the projection in the frontend.
the projection should go in the backend instead, preferably in View.
an extensive discussion about removing proj4js can be found in #345

Solution

make sure all leaflet pages are free from using proj4js.
what should be done in the backend:

• the calculation of bounding box (i.e. only the map bounding box. individual layer BBs are already available)
• the x- and y-axes for the centre of a map

the example of this solution can be found in #345

child of #457

[gubuntu]

fix this while fixing #289

[gubuntu]

@boney-bun was this fixed when #289 was fixed? Please update status