Feature/duration field

This is a new Tracker field of type Duration - allows a selection of years, months, weeks, days, hours, minutes, seconds (or any combination of these), storage, display and conversion. New Vue.js components were created for the input entry as none of the pre-made solutions made sense. Field works with and without vue.js being enabled as that is still an experimental feature. Vue.js component is much more user-friendly as it allows individual selection, +/- controls, data integrity (automatic range conversion, e.g. 25 hours become 1 day 1 hour) and many more! Uses default Tiki styles but we can easily upgrade to bootstrap components or something else. Non vue.js version is very rudimentary - a list of input fields and units. Search index contains entered value in seconds as well as the humanized string in its "_text" form.

@all any feedback would be welcome! Especially @jonnybradley, if you can review the vuejs integration I did - used your vuejslib and also the way you integrated tracker rules... let me know if anything does not fit right. This integration would give us a simple way to define vuejs components for more tracker fields and continue integrating them.