[Refactor/Feature/Data] Multipe changes as described below

- [Refactor] Merge `bind` in mod_CUsMgr with `bind` in mod_keyboardLib
- [Refactor] Split helper module into mod_commonHelper and mod_contentHelper
- [Data] Change default shortcuts for scrolling page to shift+j/k
- [Refactor] Separate mod_main module from mod_core. Among other things, this this allows mod_core (on which many other modules depend) to have as few dependencies itself as possible
- [Refactor/Feature] Rename mod_browserFunctions to mod_specialTabOperations and changed it to work using chrome.commands API
- [Feature] Introduce the concept of `protectedWebPageShortcuts` as a preferred alternative to "partially disabling" extension on a certain site
- [Refactor] Merge `browserShortcuts` with `generalShortcuts` in content script, and associated changes
- [Feature] Change the behavior of disabledSites feature to only have an true/false value (i.e. "parital disabling" is no longer required owing to the last two points)
- [Data] Add 'j' and 'k' as `protectedWebpageShortcuts for Gmail
Merge branch 'master' of https://github.com/MakeBetter/Units

Conflicts:
	background_scripts/mod_settings.js
	background_scripts/options_page/options_page.js
	content_scripts/mod_CUsMgr.js