Revert "Switched to a forked version of rails_xss that includes a fix we need"

This reverts commit 0f0b36c8883605dbe0cfc663a7b046f3a1a8d196.
Revert "Assume the result of doing any translation is html_safe"

This reverts commit 493e8e7ec43351952e2d7876afaede0cdb5d3644.
Switch from unofficial rails_xss gem to official rails plugin for rails_xss
Layouts for text emails should not escape their contents
Don't escape text emails
More small html escaping fixes
Added comment about the syntax of the interpolations in the translations
Fix translations with interpolations to correctly handle safe_html
More escaping fixes
Remove escaping of code in gettext_interpolation because it's not used and not tested
There's no way that gettext_interpolate would work when not passed a hash - untested and unused. So removing.
Removed reserved keys in gettext_interpolation because they were unused and untested