Element-R: improve logging around setting up and resetting 4S, cross-signing, and key backup

[richvdh]

Sometimes we get reports from users who have problems with some or all of cross-signing and key backup. Often they will have made attempts to reset parts of them via the UI.

It is very, very hard to tell what is going on from the logs. For example:

• During a reset attempt, which buttons did the user actually press? Which dialogs did they see?
• During normal login, we don't seem to log anything other than "Creating an empty cross signing identity stub", which is unhelpful.
• when the logs say "activated a backup", what does that mean? Will we request keys for UISIs, or upload received keys, or both?

[richvdh]

This was partly improved by https://github.com/matrix-org/matrix-react-sdk/pull/12765