Implement transaction category management (#688)

* Singularize "transaction" in transaction-nested paths

* Refactor category badge partial

* Let modal content define its width

* Add contectual menu to transactions index

* Add null_category helper

* Implement category edits

* Fix inline transaction category badges

* Fix typos in system test paths

* Add missing translations

* Add decoration to color select controller

* Wire up transaction category creation

* Fix indent in color-select-controller

* Add button for clearing category from transaction

* Implement category deletions

* Fix existing modal sizes

* Use null_category in a single place

* Remove anemic method in category deletion controller

* reassign_and_destroy -> reassign_transactions_then_destroy

* Fix i18n

* Remove destroy action from CategoriesController callbacks

* transactions_merchant -> transaction_merchant

* reassign_transactions_then_destroy -> replace_and_destroy

* Add transaction category CRUD tests

* Add presence check for transaction_id

* Check replacement_category_id presence

* Test Transaction::Category#replace_and_destroy!

app/views/transactions/_transaction.html.erb

@@ -13,7 +13,7 @@
   <% else %>
     <%= render partial: "transactions/transaction_name", locals: { name: transaction.name } %>
     <div class="w-36">
-      <%= render partial: "transactions/categories/badge", locals: transaction.category.nil? ? {} : { name: transaction.category.name, color: transaction.category.color } %>
+      <%= render partial: "transactions/categories/badge", locals: { category: transaction.category } %>
     </div>
   <% end %>
   <div class="ml-auto">