Add a clever way to generate GetLegendGraphic URL

[jacobwod]

When editing a WMS layer in Admin UI, there's an option to set legendIcon for each given layer (and sublayers):

---

While useful as-is, it can easily become cumbersome when the amount of sublayers increases:

---

However, the URL for each layer follows (oftentimes) a pattern. Normally we would like to send some options to the WMS service (e.g. to disable the labelling on icon graphic), or request some custom size of the icon.

We will probably end up with something like this:

layer1: https://somewhere.overthe.rainbown/wmsserver?SERVICE=WMS&REQUEST=GetLegendGraphic&VERSION=1.1.0&FORMAT=image/png&LAYER=layer1&LAYERTITLE=false&RULELABEL=false

layer2: https://somewhere.overthe.rainbown/wmsserver?SERVICE=WMS&REQUEST=GetLegendGraphic&VERSION=1.1.0&FORMAT=image/png&LAYER=layer2&LAYERTITLE=false&RULELABEL=false

layerN: https://somewhere.overthe.rainbown/wmsserver?SERVICE=WMS&REQUEST=GetLegendGraphic&VERSION=1.1.0&FORMAT=image/png&LAYER=layerN&LAYERTITLE=false&RULELABEL=false

We could make the administration much easier if we could expose some parameters in the Admin UI, while determine the other ones (URL to the service, layer's name) from the layer's existing definition.

[Hallbergs]

I like it! However, maybe we should hold off implementation until we're building the new Admin UI?

[jacobwod]

Definitively, as the most of it will go there - no point in doing it now!