Add a global Save button on the setting pages

### The problem

Currently in the setting pages, different sections e.g. Upstream DNS servers/DNS server configuration... has separated Save buttons. Sometimes we could make changes in different sections but forget to click the Save buttons in some sections...

### Proposed solution

There should be a global Save/Reset buttons stick on the screen whenever any changes are made in any sections. Clicking on those ones will save/reset all changes.