Insert a BOM check for import and file upload

* Similar to @janekptacijarabaci fix in greasemonkey/greasemonkey#1940
* Fix compliance with STYLEGUIDE.md and usage of pre-initialized identifiers
* Currently **do not** propagate BOM with meta routine or user.js source with and without installation count increment
* BOM currently shows up in Ace as a exclamation triangle with "This character may get silently deleted by one or more browsers"

**NOTE**: Many thanks to the report by @cvzi and applies to #200 and partially outlined in #198.