Small improvements to the 'Cylc users' page w/ map

[sadielbartholomew]

Following on from #21, there are some minor enhancements that can be made to the Cylc Users (.../users.html) page which details Cylc-using major sites, now with a world map of the sites as well as the original list.

Up for grabs if anyone fancies tackling any of these! Just assign yourself.

Sites (both listing & map of):

• Extend content-wise to include more known sites, as the list will surely have expanded since it was written up. I don't think they need necessarily be 'major' sites, as why not show off as many as possible? (Assuming the labels can accommodate them, that is!)
• (from BK in Cylc-using sites: interactive world map & nested list #21 (comment)) Derive listing from the same data structure as the map (see here), so updating would require changes in just one place & therefore would make maintenance simpler & prevent inconsistencies. (SB note: Whilst doing so, put the items in that structure in some form of order, as it is currently arbitrary, e.g. alphabetical by site name.)

World map of sites:

• Improve handling of labels so they do not overlap, but ideally via a means that does not require custom offsets relative to the plotted node on the map (as is implemented at the moment as a "good enough" solution). OpenLayers may even provide an in-built solution for this, I just could not find anything along those lines from their docs at the time.
• Have a better way to process (via the data structure) & plot (both the nodes & labels) any co-located sites, e.g. ESiWACE & DKRZ: perhaps plot with some larger or more elaborate icon, or with two nodes linked together & offset from the true location.
• (from HO in Cylc-using sites: interactive world map & nested list #21 (review)) A reset button to restore the original view after a bunch of scrolling and zooming.
• (ditto to the above) Code to prevent scrolling off the boundaries of the map.