[openstreetmap-website] Replace scale with zoom level in Export tab (#368)

[pnorman]

> So, not only image size was changed (which zoom coefficient you are proposing implies), but also a number of features present on a map extract.

Unfortunately this is going to always happen, I spent days trying to figure out how to best adjust these kind of options when writing mapbook. It also turns out it's flat out impossible with the current osm.xml style using images for some features.

I'll just quote @systemed on this

> ```01:17 <RichardF> pnorman: Mapnik's scale logic is somewhere between weird and broken... presenting sane UI for that is always going to be very hard```

I also think it doesn't matter for the immediate purposes of this pull. I think what we want to nail down first is presenting a UI for getting the number of pixels right. It may require that we also do stuff behind the scenes to make sure that the appropriate "zoom level rendering" is shown but this depends on some stylesheet stuff first.

How about this for a UI? Pardon the bad ascii-art

User selects an area and zooms in/out like now

```
    Options
------------------
Format [PNG]
Zoom  [A]X
<-----------|------>
  '  '  '  '  '  '
Image Size [C] X [D]
------------------
<Export>
```

The ``'  '  '  '`` are ticks on the slider to give a guide to where .25x, .5x, 1x, 2x, 4x, etc are.

Not 100% sold on the slider, but presenting it anyways

The user adjusts A or the slider to change the zoom which live-updates values C and D. If they type in C or D then it live-updates A, the slider and the other size field.

When the press <Export> it generates an image, which uses approximately the same cartography of the zoom level they currently have displayed. 

The last bit might need to wait and be implemented later, because doing that right likely involves changes to the backend that handles the export call.

---
Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/issues/368#issuecomment-21307159
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