keep `remove_filter` from hitting the database

Now any calls to it are cached until render time :heart:
update to #2006; better file lookup

This uses the same convention as `RAILS_VERSION_FILE` for more consistent lookup.
Specifically, #2006 broke `./script/local server`.
Merge pull request #2014 from Daxter/bugfix/1876-lazy-filter-removal

[1876] Make `remove_filter` lazy
always return a string from `with_new_form_buffer`

fixes `semantic_errors` regression from #1996 as per
my discussion with @coreyward in the diff for a1cfdb69d7ad