warn users turning on e2e in rooms with invited users that invitees won't be able to decrypt

[ara4n]

until element-hq/element-web#3821 and #647 are fixed

Other related issues:

• I should be able to read encrypted scrollback in rooms, if history visibility allows. #749
• Implement MSC3061: Sharing room keys for past messages on Invite #39
• Let devices request keys to decrypt past history for a room. #647

[MadLittleMods]

It would be good to add a warning to the top of the timeline explaining what's going on to the users experiencing the missing history (#1563)

Additionally in the settings:

• If encryption is enabled, disable the Members only (since the point in time of selecting this option) option with an info icon since it doesn't work as expected and is as good as the other option Members only (since they were invited) with the current state of things.
• If Members only (since the point in time of selecting this option) is set, then having a warning before enabling encryption and even after so when the admin goes to double-check their settings, it explains the failure case.
  • The problem with a warning is people will get themselves in a pickle no matter what warnings we show or explain. It would be good to error on the side of providing this info though. And it's better to explain things than leaving the user wondering if they did something wrong.