Added support for base href customization

[tk-nguyen]

To test, use the following commands (assuming you already have an SSH key added to your Github account):

eval $(ssh-agent -s)
ssh-add
docker build --ssh default --build-arg TWAKECHAT_BASE_HREF=<your href> -t twake-web:test .
docker run -d -p 80:80 twake-web:test

Then access http://localhost. It will auto redirect to http://localhost${TWAKECHAT_BASE_HREF}. By default, TWAKECHAT_BASE_HREF defaults to /web/

cc @nqhhdev

[github-actions]

This PR has been deployed to https://linagora.github.io/twake-on-matrix/2070