[ADDED] Possibility to disable navbar live updates

* Added new admin options: disable_navbar and disable_navbar_api
* Removes LIVE STATS from navigation list
* Removes live updates on Pool General Statistics page
* Added system load checks to getnavbardata API call

This will help to decrease load on high-volume servers at the cost of
losing live status.

Fixes #1014 once merged.
Merge pull request #1087 from TheSerapher/issue-1014

[ADDED] Possibility to disable navbar live updates