MDL-39832 Files: Revert MDL-39688 commit, 6ce0c9ce42fa7849e67d7611a628ece51c336c5f

MDL-39688 has been successful in providing a solution for the issue but further
investigations done in MDL-39832 have got the evidence that the given solution
is more likely a workaround.
MDL-39832 Files: Fixed ETag format according with RFC2616.

ETag must be double quoted,
http://tools.ietf.org/html/rfc2616#section-3.11:

      entity-tag = [ weak ] opaque-tag
      weak       = "W/"
      opaque-tag = quoted-string
MDL-40436: auth_ldap: Fixed warning when adding new LDAP users
MDL-40035: Avoid rebuild_course_cache always for empty course
MDL-40093 mod/assign: Remove count of drafts in offline assignment's grading summary screen
Merge branch 'm23_MDL-39832_Fix_Chrome_Issues_ETag_XSendfile_2ndRound' of https://github.com/scara/moodle into MOODLE_23_STABLE
Automatically generated installer lang files
Merge branch 'install_23_STABLE' of git://git.moodle.cz/moodle-install into MOODLE_23_STABLE
weekly release 2.3.8+