Add CSP to webextension manifest

Closes #80 (closed)

requested review from @meskio

We could go further with default-src 'none'; script-src: 'self', but this is better than default, so

I don't have a strong opinion here, I think is fine to keep it default-src 'self'.

Thanks!

approved this merge request

merged

mentioned in commit WofWca/snowflake-webext@a3d6e170

mentioned in commit WofWca/snowflake-webext@dce85291

mentioned in commit WofWca/snowflake-webext@f343061b

mentioned in merge request !92