feat: Config option to skip seen synchronization (#7694)

At urgent request from @hpk42, this adds a config option `team_profile`.
This option is only settable via SQLite (not exposed in the UI), and the
only thing it does is disabling synchronization of seen status.

I tested manually on my Android phone that it works.

Not straigthforward to write an automatic test, because we want to test that something
does _not_ happen (i.e. that the seen status is _not_ synchronized), and
it's not clear how long to wait before we check.

Probably it's fine to just not add a test.

This is what I tried:

```python
@pytest.mark.parametrize("team_profile", [True, False])
def test_markseen_basic(team_profile, acfactory):
    """
    Test that seen status is synchronized iff `team_profile` isn't set.
    """
    alice, bob = acfactory.get_online_accounts(2)

    # Bob sets up a second device.
    bob2 = bob.clone()
    bob2.start_io()

    alice_chat_bob = alice.create_chat(bob)
    bob.create_chat(alice)
    bob2.create_chat(alice)
    alice_chat_bob.send_text("Hello Bob!")

    message = bob.wait_for_incoming_msg()
    message2 = bob2.wait_for_incoming_msg()
    assert message2.get_snapshot().state == MessageState.IN_FRESH

    message.mark_seen()

    # PROBLEM: We're not waiting 'long enough',
    # so, the 'state == MessageState.IN_SEEN' assertion below fails
    bob.create_chat(bob).send_text("Self-sent message")
    self_sent = bob2.wait_for_msg(EventType.MSGS_CHANGED)
    assert self_sent.get_snapshot().text == "Self-sent message"

    if team_profile:
        assert message2.get_snapshot().state == MessageState.IN_FRESH
    else:
        assert message2.get_snapshot().state == MessageState.IN_SEEN
```

src/config.rs

@@ -452,6 +452,10 @@ pub enum Config {
     /// The options are from the `WhoCanCallMe` enum.
     #[strum(props(default = "1"))]
     WhoCanCallMe,
+
+    /// Experimental option denoting that the current profile is shared between multiple team members.
+    /// For now, the only effect of this option is that seen flags are not synchronized.
+    TeamProfile,
 }
 
 impl Config {