keep `remove_filter` from hitting the database

Now any calls to it are cached until render time :heart:
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
fix jQuery 1.9 compatibility issue

As per this discussion:
https://github.com/gregbell/active_admin/pull/1879#discussion_r3453478
cleanup

this is a general refactor, and doesn't change behaviour
Merge pull request #2016 from Daxter/refactor/js

JS Refactor
fixes regression from #2014

closes #2020
Use Rails' url helper in devise.rb's root_path.

This fixes #1791.