feat(insights): replace fixed overviews with a single arrangeable overview

Categories, Tags and Entities Overview are merged into one Overview page.
Levels are chips that can be dragged (or moved with the arrow keys) to set
the hierarchy and clicked to switch off, so any ordering of the three is
reachable from the same page.

Only the results reload on change; the controls stay in place. The chip
order is kept in the session and defaults to categories by tags, matching
the old Categories Overview.

app/apps/insights/utils/overview.py

@@ -8,15 +8,16 @@ from apps.currencies.models import Currency
 from apps.currencies.utils.convert import convert
 from apps.transactions.models import Transaction
 
-# Grouping levels an overview can be built from. Any ordering of these keys is a
-# valid hierarchy, which is what makes the categories, tags and entities
-# overviews the same view with a different level order.
+# Grouping levels the overview can be built from. Any ordering of these keys is
+# a valid hierarchy, which is what lets the user arrange the chain freely.
 LEVELS = {
     "categories": {"field": "category", "name": "category__name"},
     "tags": {"field": "tags", "name": "tags__name"},
     "entities": {"field": "entities", "name": "entities__name"},
 }
 
+LEVEL_KEYS = tuple(LEVELS)
+
 CURRENCY_FIELDS = (
     "account__currency",
     "account__currency__code",
@@ -234,3 +235,35 @@ def get_grouped_totals(
             )
 
     return result
+
+
+def _known(values):
+    """Drop anything that is not a level, and any repeat."""
+    levels = []
+    for value in values:
+        if value in LEVEL_KEYS and value not in levels:
+            levels.append(value)
+    return levels
+
+
+def clean_chain(values):
+    """
+    The full chain in the submitted order.
+
+    Every level always has a chip, switched on or not, so a short or unknown
+    submission is padded back out rather than rejected.
+    """
+    chain = _known(values)
+    return chain + [key for key in LEVEL_KEYS if key not in chain]
+
+
+def clean_levels(values, chain):
+    """
+    The levels actually switched on, ordered by their place in ``chain``.
+
+    Taking the order from the chain rather than from the submission keeps the
+    hierarchy and the chips in step even if the two disagree. Never empty: the
+    first chip stands in, since an overview with no levels has nothing to show.
+    """
+    enabled = set(_known(values))
+    return [key for key in chain if key in enabled] or list(chain[:1])