Read the persisted summary interval from a cookie

This cookie will be read only if the `interval` or `from` query params
are not set. If the cookie is also unset, it will still default to
the "today" interval.

TODO: The cookie still needs to be set on the client
with a `Set-Cookie` response header.

models/shared.go

@@ -12,9 +12,10 @@ import (
 )
 
 const (
-	UserKey       = "user"
-	ImprintKey    = "imprint"
-	AuthCookieKey = "wakapi_auth"
+	UserKey               = "user"
+	ImprintKey            = "imprint"
+	AuthCookieKey         = "wakapi_auth"
+	PersistentIntervalKey = "wakapi_summary_interval"
 )
 
 type MigrationFunc func(db *gorm.DB) error