IM/RTC: Perfect Forward Secrecy Requirement

[redoomed1]

@jonah

we downranked instant messengers without PFS [like Element], to account for these social-networking-like group functionalities we wanted to recommend. If we categorize these as social networks we can describe the risks/benefits of each more clearly.

With this change already reflected on the website, we can now take the first approach:

@jonah

We can require PFS, and remove Session

update!: Require PFS for instant messengers and remove Session