Sharing room keys for past messages

[manuroe]

As per MSC3061

Web PRs:

• Add function to share megolm keys for historical messages, take 2 matrix-org/matrix-js-sdk#1640
• Share keys for historical messages when inviting users to encrypted rooms matrix-org/matrix-react-sdk#5763

[manuroe]

Kamino cloned this issue to vector-im/element-android