This commit adds a setting for mailserver, that allows the client to
specify the syncing period.
Also fixes a minor issue with deletion of public chats.
This property is useful for clients to know when a channel or chat
was joined so they can use that to calculate the order of channels
and chats shown in applications.
Changes include a new joined property on the Chat struct,
as well as adjustments in the persistence layer to retreive and
update chat data in the database.
In addition there's a migration script that alters the existing
chat table to introduce a new column for the joined field.
It also updates all existing rows in the database to set `joined`
to `0`.
Andrea Maria Piana
Brian Sztamfater
Pascal Precht
Roman Volosovskyi
Volodymyr Kozieiev