Refactor TemplateEmailFile to use same sorting strategy as other models

[quis]

Other models sort based on their __sort_attribute__. This commit moves the calculation of each files position in the template into the TemplateEmailFile object, so that its sorting can also work in this way.

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The actual sort order is tested here:

notifications-admin/tests/app/main/views/test_template_email_files.py

Lines 26 to 45 in 0c85f9f

	(
	# Content order matches database order
	"((test_file_1.csv)) ((test_file_2.png))",
	["test_file_1.csv", "test_file_2.png"],
	),
	(
	# Content order does not match database order
	"((test_file_2.png)) ((test_file_1.csv))",
	["test_file_2.png", "test_file_1.csv"],
	),
	(
	# Content order does not match database order (case differs)
	"((TEST FILE 2.PNG)) ((TEST FILE 1.CSV))",
	["test_file_2.png", "test_file_1.csv"],
	),
	(
	# Content order does not match database order (extra, non-file placeholders)
	"((test_file_2.png)) ((first name)) ((last name)) ((test_file_1.csv))",
	["test_file_2.png", "test_file_1.csv"],
	),