Take anchor directives into account when computing start time in ActivityForm

[AaronPlave]

Can use getActivityDirectiveStartTimeMs from time.ts

Workaround: Refer to time in Activity Spans table

[AaronPlave]

@camargo should we be taking the anchored span duration into account here? This form is used in a bunch of places like PlanMergeReview and this start time would then depend on whichever simulation you've selected which might be odd?

[camargo]

@AaronPlave Yeah good point. In that case maybe we should go with the other option and grey out the start time all together? Maybe say to look at the anchored offset? Curious about @lklyne's thoughts on this.

[AaronPlave]

@mattdailis @dandelany @lklyne can we chat about this at some point? Might need to discuss with users.