Move layer definitions to pane controllers

[hlfan]

Closes #5616 as a partial spiritual successor of #4335

[AntonKhorev]

Is it possible to split this pull request per each sidebar pane? For example, to make a PR that moves only the map key first?

[hlfan]

Sure, the map key sidebar pane content loading update would need to be merged before the other panes' PRs as they would depend on that.

[tomhughes]

I mean it is already split into separate commits I think and I did have a quick look at it reviewing each commit but I've been meaning to come back and have a better look... As I recall my main question was whether the URLs should all be at the top level or should be under some common prefix.

[hlfan]

under some common prefix

Good point, what do you think of "/panes/{pane}"?

[hlfan]

A little bit more cleaning up the HTML by putting the layer definitions in the pane controller like how also the legend is prepared. Then I think the structure is clean enough to also close the underlying issue.

[tomhughes]

Looks good to me, thanks.