Add orthophotos for Helsinki region

[NKAmapper]

This PR adds orthophotos for the Helsinki region, including Espoo and Vantaa

In some areas they are better than the existing national orthophotos from MML, so both have been given a "best" attribute.

Please note that tile size is 512 for this wms (tile_size).

Permission: Link

[grischard]

See #522 for the tile_size attribute.

[grischard]

But if it's a wms, I'm not sure I understand the 'tile_size' attribute?

We don't have the 'valid-georeference' attribute in ELI either. Maybe we should, but that's probably for another PR.

Can you add https://storage.theel0ja.info/imgup3/Screen%20Shot%202018-08-07%20at%2019.15.29.png to the sources/europe/fi directory and link to it like we did on https://osmlab.github.io/editor-layer-index/sources/australia/au/act/ACTmapi-Imagery.PDF ?

I'm happy merging Espoo and Vantaa in the same PR too.

[NKAmapper]

Ok, done. Please ignore tile size, my mistake.

Espoo, Vantaa and Helsinki are already included in this PR, so no additional imagery needed (one combined wms for all three cities).

[grischard]

Thank you!

[NKAmapper]

#339 may be closed now :)