Session settings: remove "block new logins" option

[askmeaboutlo0m]

This option in the session settings keeps causing confusion. Some people abuse it to avoid setting a password, which is no longer necessary now that invite links include it. Some also think that it means that users who previosly joined the session can join again, but no, it just closes the session entirely.

Since the option is totally redundant with setting a password, we should just remove it altogether. If a user wants to close their session to new people joining, they can change the password and not tell anyone the new password.

The server still needs to handle old clients attempting to close the session and tell them to go set a password instead. New clients still need to handle old servers that have this option and present it in their UI if it's enabled, so that they can turn it off again. They should not be presented with it otherwise.