Merge remote-tracking branch 'upstream/master'
Added check for function apache_request_headers (since it isn't available when PHP runs as FPM).
temp pull request while FEC-2250 is fixed
updated docs for widevine test
removed unused file, removed unused variable validEventList
bump v2.21
Merge pull request #1160 from kaltura/ga-cleanup

GA: removed unused file, removed unused variable validEventList
Merge pull request #1137 from kaltura/temp-widevine-ac-branch

Temp widevine ac branch
Update CaptionsCustomVarsTTML.qunit.html

added note per incompatibility of CMML with some style rules
FEC-2312 set CORS header for JSON config POST on kgit
#FEC-1667 #time 2h #comment wait for all thumbnails to finish loading before applying resize
#FEC-405 #time 1h #comment prevent Freewheel loading on IE8. Issue a log directing to KDP usage. Updated documentation on test page.
#FEC-2313 #comment storing iframe url on kWidget
Merge pull request #1187 from kaltura/FEC-2313

#FEC-2313 #comment storing iframe url on kWidget
Added related videos and changeMedia link to GA test
cleaned up names for ForceKPlayer test file link to it from documentation
add akamai analytics forceKPlayer test
comment out log FEC-2316 is addressed for flash player
updated log message
#SUP-3132 #time 1h #comment add transparency support to buttons background color in custom style