Add group-specific "important tags" for topics

This enables (manually, so far) setting "important tags" for individual
groups. Topics in the group with any of these tags will have them shown
in listings next to the group name (similar to how spoiler and nsfw work
globally).

tildes/alembic/versions/b761d0185ca0_groups_add_important_topic_tags.py

@@ -0,0 +1,31 @@
+"""Groups: add important_topic_tags
+
+Revision ID: b761d0185ca0
+Revises: 679090fd4977
+Create Date: 2019-10-26 01:51:21.231463
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+from tildes.lib.database import TagList
+
+
+# revision identifiers, used by Alembic.
+revision = "b761d0185ca0"
+down_revision = "679090fd4977"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "groups",
+        sa.Column(
+            "important_topic_tags", TagList(), server_default="{}", nullable=False
+        ),
+    )
+
+
+def downgrade():
+    op.drop_column("groups", "important_topic_tags")

tildes/scss/modules/_topic.scss

@@ -133,6 +133,7 @@
 .topic-tags {
   display: flex;
   flex-wrap: wrap;
+  align-items: center;
   margin: 0;
 }
 

tildes/tildes/models/group/group.py

@@ -57,6 +57,9 @@ class Group(DatabaseModel):
         Boolean, nullable=False, server_default="false"
     )
     common_topic_tags: List[str] = Column(TagList, nullable=False, server_default="{}")
+    important_topic_tags: List[str] = Column(
+        TagList, nullable=False, server_default="{}"
+    )
 
     # Create a GiST index on path as well as the btree one that will be created by the
     # index=True/unique=True keyword args to Column above. The GiST index supports

tildes/tildes/models/topic/topic.py

@@ -165,13 +165,15 @@ class Topic(DatabaseModel):
         """Return only the topic's "important" tags."""
         global_important_tags = ["nsfw", "spoiler"]
 
+        important_tags = set(self.group.important_topic_tags + global_important_tags)
+
         # used with startswith() to check for sub-tags
-        global_important_prefixes = tuple([f"{tag}." for tag in global_important_tags])
+        important_prefixes = tuple([f"{tag}." for tag in important_tags])
 
         return [
             tag
             for tag in self.tags
-            if tag in global_important_tags or tag.startswith(global_important_prefixes)
+            if tag in important_tags or tag.startswith(important_prefixes)
         ]
 
     @property