this change adds the values of ESP.gteMaxAllocHeap() and
ESP.getMinFreeHead() to the prometheus metrics and the system
information object. the web UI uses these values to diplay the size of
the largest free contiguous block, calculate a rough estimate for the
level of fragmentation, and the maximum usage of heap memory since boot
in absolute and relative amounts.
The Firmware Version link now referes to the release page if the given hash is a tag. It referes to the commits page if it's really a hash. (Implements #778)
If the return value of a API request equals 401 then the user will be redirected to the login page.
Using this method it's possible to decided within the API if a authentication is needed or not.
