Remove the dimension of time from activities

[zanebclark]

This discussion is a spin out from #64

The current list of activities conflates two separate dimensions: time and activity. For example, parking and stopping are only distinguishable by time. This poses a few problems:

1. Activities now have a quasi-hierarchy where one activity implies that another activity is allowed. For example, parking also allows stopping. As additional activities are added, informing and enforcing this hierarchy will become more complicated.
2. It's possible to define a Rule with a max_stay that is lower than the duration of the allowed activity (e.g. parking).
3. What is the allowed duration for parking or stopping? Where is that defined? Is it user_class dependent?

I propose that activities with a time-based definition be removed from the list of activities (parking, stopping). To reflect these time-based activities, max_stay should be used instead. In response to the points above:

1. Quasi-hierarchy: No more hierarchy to enforce or inform
2. max_stay less than activity duration: There's no longer an overlap of responsibility here
3. Activity duration definition: Activities no longer require a duration definition. max_stay can be used to make the duration of the activity user_class-dependent.

[jlarsonOmahaNE]

@zanebclark Do you think that the activity descriptions just need to be refactored?

parking - may park and leave vehicle unattended
no parking - may not park and leave vehicle unattended
loading - loading of goods
no loading - no loading of goods
unloading - unloading of goods
no unloading - no unloading of goods
stopping - stopping briefly to pick up or drop off passengers
no stopping - stopping, loading, unloading, and parking are all prohibited; not a typical travel lane
travel - represents curbside lanes typically intended for moving vehicles, like bus lanes, bike lanes, and rush-hour-only travel lanes; implies that parking, loading, unloading, and stopping are prohibited.
no travel - no travel is allowed, implies it could be used for other activities