Update API key placeholder text throughout to "your-mapzen-api-key"

[rmglennon]

This text is used to replace the placeholder text with the user's actual API key when signed in on the website.

These need updating:

• Android - Update placeholder text, few links, typos android#404 (has issues)
• Cartography - Update api keys, typos, writing style tangrams/cartography-docs#30 (has issues)
• iOS - Update placeholder text, fix link ios#336 (OK)
• terrain tiles - Change underscores to hyphens in placeholder api key tilezen/joerd#147 (OK)
• vector tiles - Update API key placeholders, broken links, style tilezen/vector-datasource#1275 (OK) (merged, now in a release)
• Who's on First - Docs are generated, but updated to work. (OK)

These use this style, but may need some content edits, too:

• mapzen.js - https://github.com/mapzen/mapzen.js/pull/399 (OK) (merged, now in a release)
• mobility - Update API key placeholders and tutorial for mapzen.js valhalla/valhalla-docs#177 (has issues)
• search (links and tutorial) - Update text referring to replacing key placeholders pelias/documentation#199 (OK)

[rmglennon]

We should go ahead and merge the content changes. Remaining issues with the code for the swapping process are being tracked in #312.

[nvkelso]

Cartography and terrain tile changes are live on mapzen.com/documentation, but while vector-datasource has been merged to master it'll require some finagling to do a special release. Please ping me when this is closer to all going live and I'll do that release management.