MDL-39832 Files: Revert MDL-39688 commit, f111746140fe80a821ae69e3bf518d950bf188a1

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
Merge branch 'm26_MDL-39832_Fix_Chrome_Issues_ETag_XSendfile_2ndRound' of https://github.com/scara/moodle
MDL-40354 groups: Add unit tests for the api groups_group_visible()
MDL-40354 groups: Add new api groups_group_visible()
MDL-40354 groups:groups_get_activity_allowed_groups() should use passed userid to check capabilities
MDL-40354 groups: Fix issues in groups_group_visible()
Merge branch 'MDL-40354-master' of git://github.com/ankitagarwal/moodle
MDL-40449 use data generator to create random qs
MDL-40368 tool_capability: fixed regression marking single result selected
MDL-38366 repository: get_option() returns proper values

Previously, even when a specific setting had to be returned, this method
would return an empty array. Now, the behaviour is the same whether
some options or none are set: if the requested setting is not found
null is returned.
MDL-38366 repository_filesystem: Prevent use of empty subdir
Merge branch 'MDL-38366-master' of git://github.com/FMCorz/moodle
MDL-31776 - lib: mod/forums - Fixed discussion sql query.
MDL-31776 - lib: Alternate name fields - fixed up user object in question code.