Offer a configurable device name

[amandel]

The device exposes its name when opening a hotspot, exposing the bluetooth service and likely other places. This name should be part of the configuration and have a decent unique default.

Default could be:

  char deviceName[32];
  snprintf(deviceName, sizeof(deviceName), "OpenBikeSensor-%04X", (uint16_t)(ESP.getEfuseMac() >> 32));

it should be used consistently at all places, limitations might apply to make the name valid for all use-cases (use of whitespace, special chars etc.).

The name should also be part of the page title of the exposed pages of the html configuration server. So that it is used by the browser too,