core: group description (#1538)

* core: group description

* support multi-line welcome message

* fix

src/Simplex/Chat/Migrations/chat_schema.sql

@@ -116,7 +116,8 @@ CREATE TABLE group_profiles(
   updated_at TEXT CHECK(updated_at NOT NULL),
   image TEXT,
   user_id INTEGER DEFAULT NULL REFERENCES users ON DELETE CASCADE,
-  preferences TEXT
+  preferences TEXT,
+  description TEXT NULL
 );
 CREATE TABLE groups(
   group_id INTEGER PRIMARY KEY, -- local group ID