server : support for saving templates in browser LocalStorage (#2486)

* support for templates in browser LocalStorage

* sync accepted #2409 fix from upstream

* convert autosave invocation to useEffect

* Apply suggestions from code review

Co-authored-by: Jhen-Jie Hong <iainst0409@gmail.com>

* Regen index.html.cpp, suggested from code review

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Co-authored-by: Jhen-Jie Hong <iainst0409@gmail.com>

Author: staviq