Add content security policy

[tusbar]

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[buildsize]

File name	Previous Size	New Size	Change
manifest.json	745 bytes	828 bytes	83 bytes (11%)
messages.json	1.12 KB	1.12 KB	0 bytes (0%)
background.html	273 bytes	273 bytes	0 bytes (0%)
popup.html	431 bytes	431 bytes	0 bytes (0%)
arma-128.png	8.99 KB	8.99 KB	0 bytes (0%)
arma-48-gs.png	2.04 KB	2.04 KB	0 bytes (0%)
arma-48.png	2.94 KB	2.94 KB	0 bytes (0%)
arma-64.png	4.14 KB	4.14 KB	0 bytes (0%)
arma-96-gs.png	4.67 KB	4.67 KB	0 bytes (0%)
arma-96.png	6.15 KB	6.15 KB	0 bytes (0%)
background.[hash].js	9.18 KB	9.18 KB	0 bytes (0%)
common.[hash].js	15.13 KB	15.13 KB	0 bytes (0%)
popup.[hash].js	160.89 KB	160.89 KB	0 bytes (0%)