Eslint: Add additioanl rules, some for GCI.

This commit adds rules for GCI, turns on rules that do not error
on our codebase, and changes frontend_tests/.eslintrc.json's no-sync
rule to off (as per AirBnB's style guide).

Rules for GCI:
no-restricted-syntax, no-nested-ternary, spaced-comment,
space-infix-ops, newline-per-chained-call, padded-blocks,
no-whitespace-before-property, space-in-parens

Rules that do not error:
no-useless-constructor, no-dupe-class-members, no-duplicate-imports,
no-iterator, no-undef, dot-notation, no-case-declarations, no-unneeded-ternary,
eol-last,

Finally, eqeqeq is changed from 2 to ['error', 'allow-null'], going
from jslint defaults to airbnb's recommendation (there were no errors)

frontend_tests/.eslintrc.json

@@ -11,6 +11,6 @@
 	"window": false
     },
     "rules": {
-	"no-sync": 1
+	"no-sync": 0
     }
 }