Extract template / csv utility code into modules

This follows a similar approach to the previous commits, noting that
one module depends on the other, so we have to extract both together.

app/main/views/uploads.py

@@ -46,19 +46,17 @@ from app.s3_client.s3_letter_upload_client import (
 )
 from app.template_previews import TemplatePreview, sanitise_letter
 from app.utils import (
-    Spreadsheet,
     generate_next_dict,
     generate_previous_dict,
-    get_errors_for_csv,
     get_page_from_request,
-    get_sample_template,
-    get_template,
     unicode_truncate,
 )
+from app.utils.csv import Spreadsheet, get_errors_for_csv
 from app.utils.letters import (
     get_letter_printing_statement,
     get_letter_validation_error,
 )
+from app.utils.templates import get_sample_template, get_template
 from app.utils.user import user_has_permissions
 
 MAX_FILE_UPLOAD_SIZE = 2 * 1024 * 1024  # 2MB