Added new preferences dialog to allow dynamic changing of time zone.

[michael-gebis]

• The main change in this pull request is the ability to set the time zone through a new "set-preferences" button in the web interface
• The preferences are saved in non-volatile memory, and are retained across power cycling.
• The time zone may now be a fractional hour offset
• Setting and unsetting daylight saving time is now a single checkbox
• Added a workaround to an issue where the hostname may not be set (at least in the latest non-beta arduino ESP32 libraries (V2.0.14))
• Updated README.md to include info on the above changes

[michael-gebis]

Harshit, thank you for your time on the original project, and also any time you spend reviewing my changes. One other request: I noticed this repo does not have a LICENSE file. I would like to ask you to please add one--hopefully something like the MIT license, but anything open source would be awesome. Thanks!