Rename the "Decentralization" score to "Validation" score

Live preview: http://bitcointest1.us.to/en/choose-your-wallet

Based on discussions #601, this pull request renames decentralization scores to avoid confusion, since this score is about payment validation, and centralization / decentralization is broader and depends on other factors. 

I used a mix of the propositions that have been made and especially like this being an opportunity to use seperate titles for SPV and Electrum (thanks @harding).

I don't know if there was already enough agreements on that, so I'm leaving some time for comments.

Relevant changes are in `_translations/en.yml` and `README.md`, the rest is just reorganization noise.