[Tagging] isced:level

[Fredrik]

The proposal 
(https://wiki.openstreetmap.org/wiki/Proposed_features/ISCED) for 
isced:level was marked as abandoned after 6 years of no real conclusion, 
but they key is used ~120,000 times so I created the page  
https://wiki.openstreetmap.org/wiki/Key:isced:level and copied over the 
central information from the proposal page.

Any additions, suggestions or objections?

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FredrikLindseth