server : add loading html page while model is loading (#9468)

* Adding loading page for '/' server requests

* set content when model is loading

* removed loading html file

* updated cmakelist

* updated makefile

* cleaned up whitespace

* cleanup for PR removed error

* updated server test to handle 503 HTML

* updated server test to handle 503 HTML

* ca†ch 503 before parsing json

* revert test

* account for both api and web browser requests

* precommit corrections

* eol fix

* revert changes to pre-commit

* removed print statement

* made loading message more descriptive

* also support .html files

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Co-authored-by: VJHack <flymyplane21@gmail.com>
Co-authored-by: Vinesh Janarthanan <36610342+VJHack@users.noreply.github.com>