Add ESP32 support, changed layout of inline HTML, small fixes

[Jeroen88]

Added ESP32 support for the main files PersWiFiManager.h and PersWiFiManager.cpp using #if defined(ESP8266) and defined(ESP32) to make the difference.

Added ESP32 support for the first example basic_rest_api.ino using the same #if defined's.
To make this example work you need to install the ESP32 port of the ESP8266SSDP library too. This port is called ESP32SSDP and can be found here.

The other two examples can be adapted easily, although spiffs_rest_api_nonblocking.ino requires adaptation of the EasySSDP and SPIFFSReadServer files too.

Other changes are, in basic_rest_api.ino:

• moved SSDP.setDeviceType("upnp:rootdevice") to before SSDP.begin(), because it seems more logical to fully initialise SSDP before starting it

and in PersWiFiManager.cpp:

• changed the layout of the inline HTML for const char wifi_htm[] to make it readable in the Arduino SDK
• used the ESP32 equivalent for ENC_TYPE_NONE, i.e. WIFI_AUTH_OPEN
• added HTTP headers Cache-Control and Expires for the route "/wifi.htm", both for the ESP32 and for the ESP8266, to avoid this page being loaded from the browser cache when a (next) connection to the ESP Access Point fails
• added WiFi.mode(WIFI_STA) in PersWiFiManager::begin(), because otherwise my ESP32 crashed. Maybe it is a good idea to add this for the ESP8266 version too

[Jeroen88]

Changed Access Point IP address to 192.168.4.1 instead of 192.168.1.1, because that is what the ESP32 reported back even if I set it to 192.168.1.1.

[Jeroen88]

If no WiFi credentials are found (e.g. when the ESP has never connected to a WiFi network before or the credentials have been reset) do not wait for a timeout but start the AP inmediately

[Jeroen88]

Fixed a bug in starting the AP inmediately if no WiFi credentials are found.

[Jeroen88]

On an ESP32 it was more difficult to detect if no WiFi credentials are present. This feature is not tested yet on an ESP32 completely fresh out of the box, neither is it tested yet on an ESP8266.

[Jeroen88]

Changed the DNS server IP to 192.168.4.1 too, so all clients connecting in AP mode are redirected to the ESP webserver root.

[Jeroen88]

Tested on a 'fresh out of the box' ESP32 and works like a charm!
Tested on a previously used before ESP8266 and, after the last commit, works like a charm too!

[Jeroen88]

Added a method ::resetSettings() to clear WiFi credentials, e.g. for testing purposes. If ::resetSettings() is called before ::begin(), the ESP should start in AP mode, because it has no more a known SSID to connect to.

[Jeroen88]

Some more changes:

• It seems that the ESP8266 can be (rarely) bricked if connecting to a new SSID while it is already connected to another SSID. So calling the recently added ::resetSettings() to clear a previous SSID
• The literal HTML string was wrong
• Added ::getSsid() to get the last SSID the ESP connected to, or tried to connect to

[r-downing]

Very cool, thanks!