[ESLint] autofix style and minor issues

[luclu]

Applies following fixes among others across the code-base:

• intend
• max. 2 empty lines
• spaces after keywords
• use const/let instead of var (in the appropriate context)
• prefer arrow callbacks

Basically applies all fixable rules (wrench-icon) enabled in airbnb-base config.

Travis and local builds / dev runs tested.

[mention-bot]

@luclu, thanks for your PR! By analyzing the history of the files in this pull request, we identified @alexvandesande, @evertonfraga and @hiddentao to be potential reviewers.

[lock]

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