New default channel option: Dynamically select channel based on inbox currently being viewed

If the 'Default channel' preference is left blank, the default channel selected for a new conversation is dynamically selected based on the inbox currently being viewed.

This is a useful behaviour, but it is fairly blank since it is only applied if the preference is left blank. Furthermore, if a user adds an individual channel, we make that channel their default channel automatically. This means that if an admin wishes to leave the default channel blank when inviting a teammate from a teammate template, then the blank state will be overwritten.

If we add a new explicit option for default channel 'Dynamically select channel based on current inbox' then teammate's can discover this preference and admins can select it for a teammate template.