add preferred lonely_children extension (#942)

* add preferred lonely_children extension

* simplify sampling process, lower the probability to 25%

* exclude parents for replies that were recently used

* lonely children := count > 0

* consider only tasks not done for parent exclusion

* increase lonely child sampling probability

backend/alembic/versions/2023_01_26_1835-c84fcd6900dc_add_task_created_date_index.py

@@ -0,0 +1,26 @@
+"""add task created date index
+
+Revision ID: c84fcd6900dc
+Revises: 40ed93df0ed5
+Create Date: 2023-01-26 18:35:43.061589
+
+"""
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "c84fcd6900dc"
+down_revision = "40ed93df0ed5"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_index(op.f("ix_task_created_date"), "task", ["created_date"], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f("ix_task_created_date"), table_name="task")
+    # ### end Alembic commands ###