feature: Support for up/down detection of ping only devices.

* Added support for ping only devices.

* Renamed sql-schema/206 to 207.

* Discovery tried to detect the OS when it shouldn't.

* Updated db_schema.yaml

* Added ping icon.

* Don't show unused graphs and tabs when SNMP is disabled.

* Allow the user to specify OS of ping only devices.

* Removing custom OS now changes it to 'ping'.

* Removed unnecessary use of mres().

* UI select box for SNMP enable/disable.

* Hide device_ping_perf if ping is disable on device.

* Fixed SNMP settings update status messages.

* Added functionality to add ping only devices via the web ui.

* Added ping only option to addhost.php

* Added ping only support to snmp-scan.py

* Moved sql 208 to 211

* Fixed scrutinizer issues.

* Fixed broken ossearch.

* Added ping only support to the API.

* Updated API doc.

* Added (optional) to OS and hardware description. Hid Port Settings, Applications, Modules, Storage, Processors, Memory and Components from the edit menu

* Style fix.

* Updated ping icon.

* clean() instead of mres(). More escaping. Better help in snmp-scan.py and addhost.php

* Fixed scrutinizer issue.

* Always try SNMP in snmp-scan.py, new option for it in addhost.php. Slice instead of chunk in ajax_ossuggest.php. Other minor style changes.

* Updated sql modifications to insert the new column in the same place as in db_schema.yaml.

html/images/os/ping.svg

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