CSV Transaction Imports (#708)

Introduces a basic CSV import module for bulk-importing account transactions.

Changes include:

- User can load a CSV
- User can configure the column mappings for a CSV
- Imported CSV shows invalid cells
- User can clean up their data directly in the UI
- User can see a preview of the import rows and confirm import
- Layout refactor + Import nav stepper
- System test stability improvements

Gemfile.lock

@@ -156,6 +156,7 @@ GEM
       bigdecimal
       rexml
     crass (1.0.6)
+    csv (3.2.8)
     date (3.3.4)
     debug (1.9.2)
       irb (~> 1.10)
@@ -456,6 +457,7 @@ DEPENDENCIES
   brakeman
   capybara
   climate_control
+  csv
   debug
   dotenv-rails
   erb_lint