Add nonce to css/js tags

[oz-tal]

Ensure css/js tags comply with the CSP nonce (if one is defined).

In the absence of a nonce (or the whole policy itself even), theses changes will gracefully do nothing.

[coveralls]

coverage: 96.06%. remained the same
when pulling 5da5d1b on oz-tal:csp-nonce
into 1b2a8ba on railsadminteam:master.