Web is inconsistent with Element X in terms of key storage setup

[BillCarsonFr]

The problem

Examples:

• Web does not set up key backup until the user sets up 4S ("set up secure backup"), whereas Element X sets up key backup automatically on login, even if 4S does not exist (there is an opt-out in settings).

• Web does not observe the opt out m.org.matrix.custom.backup_disabled in account data - instead if you "set up secure backup" it sets up key backup and 4S.

(See https://github.com/element-hq/element-internal/issues/516 )

One failure mode:

1. Register an account on Element Web and do not "set up secure backup"
2. Log in on Element X (which creates key backup, we think, but no 4S)
3. The keys Web has already received and received in future are never saved to key backup.
4. If on Web you do "set up secure backup" we are not sure what happens.

What we should do

• Web should always set up key backup on login
• Web should observe the opt out in m.org.matrix.custom.backup_disabled in Account data.

[andybalaam]

@BillCarsonFr is this part of the Trust work? Should it be inside this epic? https://github.com/element-hq/element-internal/issues/540

[BillCarsonFr]

@BillCarsonFr is this part of the Trust work? Should it be inside this epic? element-hq/element-internal#540

It's part of the Post Force Verification improvement task element-hq/crypto-internal#306 (moved out from element-hq/crypto-internal#283)