Allow panel/map boundary to be dragged

[mvl22]

Would be useful for the panel/map boundary to be draggable, with cookie memory, so users wanting more map space can obtain this.

Request in nptscot/nptscot.github.io#311 (comment)

Will need the panel elements to resize accordingly also.

NB On mobile the layout adjusts completely so this does not apply to that.

[Robinlovelace]

👍 thanks for reviving! Requested previously as far as I recall, see some more on this here for example: #13 (comment)