add begin(port) to esp8266webserver, move some strings to flash, some refactoring (#4148)

* add begin(port) to esp8266webserver, move some strings to flash, some refactoring

* Moved more strings to flash, unified some strings

* move mimetable strings into a standalone file

* more string moves to flash, remove duplicates, refactor of template method, minor styling

* Reverted moving small string to flash (no heap advantage, reduces bin size)

Author: devyte