Make timeframes respect chronology

Timeframes are currently treated as strings or labels within Aha!. As a result, ordering timelines will order them alphanumerically rather than chronologically. 

To a certain extent this can be worked around by specifying the year first and then the half or quarter, "2018 Q1", "2018 Q2" etc.

However, being able to specify an actual period of time (much like the external date of a release) and having Aha! respect and order this chronologically would be helpful especially when some teams use half years and others use quarters (or some other denomination) within the same Aha! account.