Improve flash-api

[phatpaul]

I made some additions/improvements to the cgiFlash.c and improved the html/js GUI for managing partitions. I'm not bothering to support the older ESP8266 chip (I don't have one).

Tested on:

• ESP32-EVB from Olimex (ESP-WROOM-32 module, 4M flash)
• Custom board with ESP32-WROVER (with 16M flash, 4.5M ram)

Note these additions complement my additions to the esphttpd-freertos repository.

The API is documented here: https://github.com/phatpaul/libesphttpd/blob/master/README-flash_api.md

BTW, I'm compressing with yui-compressor and it's working OK.

• I can't seem to call java from inside the SDK, but I hacked it by copy java.exe from the jre install directory ("C:\Program Files\Java\jre1.8.0_181\bin\java.exe") to inside the SDK at /home/username/Tools/.
• Copy yuicompressor-2.4.8.jar from https://github.com/yui/yuicompressor/releases to /home/username/Tools/
• Enable the option ESPHTTPD_USEYUICOMPRESSOR in menuconfig
• Can you find a cleaner way to do this?
• I would really like to use "webpack" https://webpack.js.org/ but it has been giving me trouble, probably because the version of node.js in the SDK is old.

[phatpaul]

I made a new branch on my fork with squashed changes that I cherry-picked. I had to make a new PR #47 since I can't seem to change the "from" branch on this PR.

Closing this PR.