Some iteration semantics got mixed up when replacing remaining uses of
`var` variable declarations in ae37fb6c.

`var foo in bar` cannot be replaced with `const foo of bar` because the
first will assign the iteration index of the current item to foo,
whereas the second will assign the actual iteration item.

* replace all `var` declarations and use `const` where appropriate
* strip unnecessary temporary variables
* replace single character variable names
* replace multiline variable declarations
* simplify expressions
* replace invalid uses of parseInt

New linter rules:
* curly
* block-spacing
* keyword-spacing

Also apply all autofixes introduced by these rules

* add filesize npm package
* add prettyNanoseconds mixin function

includes:
* new pretty date functions
* fetching of timeslots through getTimeslots function

now prepared for pagination, has to be implemented next

includes:
* new pretty date functions
* fetching of timeslots through getTimeslots function

now prepared for pagination, has to be implemented next