Remove manual hacks from mediaelement js/css

Rather than running our script that stitches together mediaelement js
and css files and _then_ manually editing in our hacks, enhance the
build script to be able to pull specific customizations from local
files.

Use this new facility to stitch in our "upload subtitle" feature in
more cleanly.

Also normalize the whitespace in our hacks to match me.js tab style to
reduce the diff between our code and upstream.

Note that the diff when ignoring whitespace on
public/javascripts/vendor/mediaelement-and-player.js is only comments.

On the CSS side, we pull out the full screen subtitle font-size bump
into a separate file. We also update the target location for the
vendored CSS file in the build script.

Now `rake build_media_element_js` should be idempotent.

Test plan:

 - Verify the media player works for video and audio.
 - Verify that captions in full screen videos show with a large font

Change-Id: I698b0c3454c12bfa4efffc2d98ab9056d351220c
Reviewed-on: https://gerrit.instructure.com/38455
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Paul Hinze <paulh@instructure.com>