remove rails dependency from html_text_helper

test plan
- ensure html text escaping still works

Change-Id: I2524f9df977a4139006030be33dc3be7db8d5f0f
Reviewed-on: https://gerrit.instructure.com/32973
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>