Challenge Topics (#1966)

* Closes #1897 
* Adds Topics to Challenges where Topics are admin-only visible tags about challenges
* Adds `/api/v1/topics` and `/api/v1/challenges/[challenge_id]/topics` to API 
* Challenge comments have been moved into a modal

migrations/versions/ef87d69ec29a_add_topics_and_challenge_topics_tables.py

@@ -0,0 +1,46 @@
+"""Add topics and challenge_topics tables
+
+Revision ID: ef87d69ec29a
+Revises: 07dfbe5e1edc
+Create Date: 2021-07-29 23:22:39.345426
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "ef87d69ec29a"
+down_revision = "07dfbe5e1edc"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table(
+        "topics",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("value", sa.String(length=255), nullable=True),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("value"),
+    )
+    op.create_table(
+        "challenge_topics",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("challenge_id", sa.Integer(), nullable=True),
+        sa.Column("topic_id", sa.Integer(), nullable=True),
+        sa.ForeignKeyConstraint(
+            ["challenge_id"], ["challenges.id"], ondelete="CASCADE"
+        ),
+        sa.ForeignKeyConstraint(["topic_id"], ["topics.id"], ondelete="CASCADE"),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_table("challenge_topics")
+    op.drop_table("topics")
+    # ### end Alembic commands ###