Expand/Collapse Files and Blob Excerpt while Reviewing/Comparing code (#8924)

Expand/Collapse Files and Blob Excerpt while Reviewing/Comparing code (#8924)

* update #8659 fold/unfold code diffs

* add fold button style

* update #8659 implement expand up/down codes (blob excerpt)

* fix golint errors

* fix expand direction

* remove debug message

* update css style for blob exceprt

* fix typo in comment

* update style sheet with less

* update expect diff (add SectionInfo)

* update #8942 accept suggested change (fix typo)

* close reader and check file type before get tail section

* adjust button position and check file type before insert fold button

* move index js to web_src

* merge index.js with master

* generate index.js

* update js coding style