refactor(data): describe tile budgets per period, drop a duplicate test, add the changelog entry

changelog.d/762-tile-budget-module.md

@@ -0,0 +1,4 @@
+- Move the tile budget helpers `utcDayKey`, `normalizeBudget` and
+  `isOverBudget` out of the TomTom module into `src/data/tileBudget.js`, so
+  other keyed tile proxies can share them. `src/data/tomtomTiles.js`
+  re-exports them; behaviour does not change (monwiss, #762).