As a department head creating shifts, I want to be able to specify shift lengths with at least 15-minute granularity

[Travis-Snoozy]

Hour-long-only shift durations (even with "+15" specifiers) are too coarse-grained for all cases. Case in point, our main stage headliners had 1.5 hour sets this year. I wanted to -- but was unable to -- create exactly-matching per-act shifts for operating the sound and lightboards. Likewise, for stagehands, I wanted to make sure that stagehand shifts ended in the middle of acts, instead of at changeover points -- meaning that I was left having to either overlap shifts or have gaps if I wanted to achieve this after the acts stopped being on hour boundaries.