Add language override feature

[L1Q]

Hopefully resolves #228

• refactor Messages.js
• expose refreshLang for easy translations reloading
• update settings version with new setting
• update settings page with new option
• make all pages refresh language before rendering

No need to reload extension! P.S.: hacktoberfest tag or label much appreciated

[schmich]

Awesome, this looks promising.

One issue: Messages.js is actually dynamically generated from messages.json (see Makefile and this script). My fault for not putting a banner at the top of Messages.js saying that it was generated by a tool. In general, I'd like to not have to maintain message IDs in multiple places.

Hacktoberfest label applied.