Bug 36531: Serve text/javascript compressed, like application/javascript is

Bug 36531: Serve text/javascript compressed, like application/javascript is

Koha serves static .js files as application/javascript (if /etc/mime.types
says to) and serves them compressed, but output_with_http_headers uses the
currently-correct text/javascript mimetype, and Koha doesn't compress that.

Test plan:
1. Set the preference EnableAdvancedCatalogingEditor to Enable.
2. Open the browser Web Developer Tools to the Network tab
3. Load Cataloging - Advanced editor
4. Click on the line for the framework?frameworkcode=&callback=define load
5. Note the content-type text/javascript, no Content-Encoding line, and
   the size of 1.9MB
6. Apply the patches from bug 36463 if they haven't been pushed, then this
   patch, and reset_all
7. Repeat steps 1-4, and note a Content-Encoding: gzip header and a
   Transferred size around 160KB

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit c52af2cef0d79f20d74ebf170d4d5514b139ea65)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>