Convert all message senders after migration to key-contacts

[link2xt]

In PR #6796 migration converting from_id of encrypted messages is limited to 10000 messages:

core/src/sql/migrations.rs

Line 1844 in 2ee3675

ORDER BY id DESC LIMIT 10000",

This is done to avoid migration taking too long (30 seconds or more) as it can be a problem on mobile devices, especially with multiple profiles.

If after migration you scroll some old chat up, the messages have from_id pointing to address-contact and have generic "envelope" avatar. This is also visible if you search for old messages.

A way to fix it would be to save the contact mapping and later convert remaining messages in background, maybe in chunks of 1000 messages as well, remembering the lowest unconverted ID.