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!

test/controllers/transactions/categories/deletions_controller_test.rb

@@ -0,0 +1,40 @@
+require "test_helper"
+
+class Transactions::Categories::DeletionsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in users(:family_admin)
+    @category = transaction_categories(:food_and_drink)
+  end
+
+  test "new" do
+    get new_transaction_category_deletion_url(@category)
+    assert_response :success
+  end
+
+  test "create with replacement" do
+    replacement_category = transaction_categories(:income)
+
+    assert_not_empty @category.transactions
+
+    assert_difference "Transaction::Category.count", -1 do
+      assert_difference "replacement_category.transactions.count", @category.transactions.count do
+        post transaction_category_deletions_url(@category),
+          params: { replacement_category_id: replacement_category.id }
+      end
+    end
+
+    assert_redirected_to transactions_url
+  end
+
+  test "create without replacement" do
+    assert_not_empty @category.transactions
+
+    assert_difference "Transaction::Category.count", -1 do
+      assert_difference "Transaction.where(category: nil).count", @category.transactions.count do
+        post transaction_category_deletions_url(@category)
+      end
+    end
+
+    assert_redirected_to transactions_url
+  end
+end