CoffeeScript tests migration: Rename files

Step 1 in our migration steps is to rename the CoffeeScript files to
the JS files they will be replaced with, so that we can preserve file
history.

cms/static/coffee/.gitignore

-*.js

common/lib/capa/capa/tests/test_files/js/.gitignore

-test_problem_display.js
-test_problem_generator.js
-test_problem_grader.js
-xproblem.js
\ No newline at end of file

common/lib/xmodule/xmodule/js/spec/.gitignore

-*.js
-
-# Tests for video are written in pure JavaScript.
-!video/*.js
-
-# Tests for Time are written in pure JavaScript.
-!time_spec.js
-!collapsible_spec.js
-!xmodule_spec.js
-!sequence/display_spec.js

common/static/coffee/spec/.gitignore

-*.js

lms/static/coffee/.gitignore

-*.js