Add automatic tslint fixers

[lukastaegert]

This will add tslint to our setup and to the commit hook. However, as we already have prettier, this PR solely introduces some hand-picked semantic rules that prettier cannot cover:

• prefer-const: This replaces let with const if possible. const is usually slightly faster in V8
• ordered-imports: Probably the biggest diff. Imports are sorted so that similar imports are next to each other and removing and re-adding an import will put it in the same position.
• arrow-return-shorthand: Replaces ()=>{return 1} with ()=>1.
• no-string-literal: Replaces x['y'] with x.y which should be better optimized
• object-literal-shorthand: Replaces {x: x} with {x}
• no-unnecessary-type-assertion: Removes all unnecessary TS type casts!
• interface-over-type-literal: Replaces types with interfaces (which are e.g. extensible) wherever possible

All of these rules have automatic fixers and are applied automatically on commit. I have put the application of the rules into a separate commit that can be easily undone so that we can remove/add rules.

[guybedford]

Nice!

Looking through, wondering if we should include no-unused-variable or prefer-object-spread as well.

[lukastaegert]

Good idea, added!