Search your encrypted emails without giving Proton Mail access

The original title was

> Search message content

which I found does not capture the significance of the blog post, so I took text from the blog post and simplified it to make a title appropriate for HN.

> Everyone who subscribes to a paid Proton Mail plan can use content search.

FWIW, I am a non-paid user and I have full content search enabled. I'm not sure if this is a bug or some A/B testing.