Better handle tail trimming

[samcrawford]

At the moment, tail trimming is enabled by default and there's no way for a user to turn it off.

This can lead to unexpected behaviour for users when editing a round trip route. Dragging a new waypoint that would result in an out-and-back onto the route of <300m will result in the user's chosen location being ignored and the point snapping back to the trimmed location. This feels weird as a user.

Ideally we would not trim tails when a user is making edits.

This will likely mean that tail trimming is no longer a global on/off option for a route, but instead becomes a per-segment option.