Change sharedHistory flag to unstable variant

[Anderas]

Resolves element-hq/element-ios#4947 which is an implementation of matrix-org/matrix-spec-proposals#3061

Handle several edge cases when key sharing:

• use unstable parameter name of shared_history when exporting and importing keys. This makes it compatible with the existing web implementation (e.g. backup keys on the web and restore on iOS => preserve sharedHistory flag)
• add integration tests for key sharing across several devices
• ensure that keys are only shared if the roomId of the session matches the room a user is invited into

[aringenbach]

Code LGTM (would be awesome to write new tests in Swift if possible though)

[Anderas]

Code LGTM (would be awesome to write new tests in Swift if possible though)

Yea in this case I wanted to write the integration tests consistently with the rest of the CryptoSharingTests, but should start switching over to Swift there as well.