[Toaster] [review-request] 7379 Download task log button for tasks with cached outcome

[Lerner, Dave]

http://git.yoctoproject.org/cgit/cgit.cgi/poky-contrib/log/?h=lerner/task_log_btn_7379

This copies the logic for executed tasks that creates a "Download task log" button, but uses the "_setscene" log for that task, which the original code used.

Testing
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1) Follow steps in defect...
Build an image.
Delete tmp directory.
rebuild and find a task from task list that says cache, like m4-native do_populate_sysroots.
Verify there is a button with 'Download task log'.
Click button.
Verify that contents of downloaded file refers to sstate_unpack_package and also refers to staging files for the package m4-native.

2) Run page through w3c html validator 
Verify that only the two unrelated errors reported (in the build pulldown).