Add Hound/Rubocop config; Make Ruby style consistent

**Analog to opf/openproject#1879 and opf/openproject#2136**
Shadowing outer local variable - `s`.<br>Line is too long. [211/80]<br>Prefer `map` over `collect`.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.