Add threshold to steps warning

[graue]

Now that there's a warning about taking your bike on steps (since #264), I'm finding the warning to be a little overzealous and spammy.

Example 1: Routes involving Glen Park BART have the warning because it thinks there are a few steps at some point. There may be a mapping issue here: bikehopper/graphhopper#126

Example 2: Routing to the Oak Woodlands in Golden Gate Park also currently has the steps warning, because the specific point it chooses seems to be along a portion of trail with steps. This could be mitigated by better geocoding to route to an entrance of a park area, not its centroid (see #156; in this case, the mapping would also need to be changed in OSM because Oak Woodlands is currently mapped as a node and it should probably be an area way).

So it might help to only give the steps warning if there are more than a few steps -- if, let's say, the total distance of "steps" is more than 10 feet, to exclude trivial cases.

I'm not sure it's possible to set a threshold that filters out trivial examples like these while keeping legitimate warnings, so if it turns out upon further investigation that it's not, this issue can be closed.