disable creation of new encrypted accounts, fade out existing ones

[r10s]

"encrypted accounts" are flagged as being "experimental" and currently cause several issues:

• 0xdead10cc crashes because of database cache files left open (unencrypted databases cache files are recognised by iOS and are okay); this adds issues to notifications as we're called less often if we crash on background handlers that shall return fast

• potential issues on locked devices as the key needed for decryption may not be available; this will add issues for notifications

• slowness on opening accounts (might be on purpose for security, to avoid brute forcing); again, this is an issue for notifications as we cannot return fast from background handlers

of course, we could also try to fix the issues, however, these resources would be missed elsewhere. also, encrypted accounts add very limited security benefits on iOS, if at all. when they were added as an experiment, it was not clear they will cause lots of problems.

actionable items:

• strike option to create encrypted accounts

• the option to open existing encrypted accounts will stay for now

• add a device message for existing encrypted accounts" saying that this will cause issues and is no longer supported. say how to convert encrypted to unencrypted account (backup export -> import, doing this automatically is not needed for experimental features, the option was always a bit hidden, i do not expect that to have many users)

[link2xt]

There is also an option to make SQLCipher databases recognizable as SQLite: https://www.zetetic.net/sqlcipher/sqlcipher-api/#cipher_plaintext_header_size
This option seems to have been added at the Signal's request: sqlcipher/sqlcipher#255
Adding this mostly for reference as SQLCipher docs document that there is an exception for locked SQLite files on iOS.

[r10s]

thanks for summing up things up. however, let's reconsider supporting encrypted accounts on iOS once we have notifications ready, working pull and at best also push (which may still cause issues wrt locking and performance on opening) (only 1 of 3+ issues would be fixed by cipher_plaintext_header_size :)

in between, it is better to disable that and to let ppl move away from encrypted account, clearly saying, it is unsupported. i'd not put our few resources into tweaking encrypted accounts (also, at it is questionable wrt added security at all)