Rename nav2_tasks (fixes #821) (#891)

* Rename nav2_tasks

Now that ROS2 actions have been implementation and the temporary tasks implementation
is obsolete, all of the task-related code has been removed from the repo. What's
left in the nav2_tasks directory is Behavior Tree related code. So, this PR renames
nav2_tasks to nav2_behavior_tree. This new directory will contain any of our
domain-specific nodes and BT-related utility classes.

* Fix a couple more cpplint/uncrustify issues

* Fix a rebase error