Use Relative URL In `templates/index.html`

[hwzh4640]

Use relative URLs in templates/index.html so that the server can run on any address besides localhost.

For examples, change:
<input type="button" value="System Overview" onclick="set_content('http://localhost:6060/system-overview')">

to:
<input type="button" value="System Overview" onclick="set_content('/system-overview')">