Stop manually slugifying PLC course names

[islemaster]

PLC-452: Some of our PLC course links (including breadcrumbs) are broken (Slack) because we've been manually slug-ifying the PLC course names for our URLs with:

name.downcase.tr(' ', '-')

...and trying to restore them with...

name.tr('-', '_').titleize

This worked fine for the one simple test case we had in place (My Plc <=> my_plc) but our real course names don't look like that. A recent broken example:

# The real course name
"CS Discoveries Deeper Learning 2019 - 2020"
# Slug-ified
"cs-discoveries-deeper-learning-2019---2020"
# Unsuccessfully restored
"Cs Discoveries Deeper Learning 2019   2020"

This PR removes the "slugification" code and replaces it with a more typical use of the course_path helper, passing it a Course model and letting it perform all the appropriate URL encoding so that the links work fine when they're used later.

I've left the following "un-slug" code in place for now, in case there are any links or bookmarks out there still depending on the old behavior:

code-dot-org/dashboard/app/controllers/courses_controller.rb

Lines 49 to 57 in 1838c6e

	course = Course.get_from_cache(params[:course_name])
	unless course
	# PLC courses have different ways of getting to name. ideally this goes
	# away eventually
	course_name = params[:course_name].tr('-', '_').titleize
	course = Course.get_from_cache(course_name)
	# only support this alternative course name for plc courses
	raise ActiveRecord::RecordNotFound unless course.try(:plc_course)
	end

We'll watch for remaining uses of this path and remove it when we're confident it's no longer needed.

[breville]

Nice!

[maddiedierker]

yay for doing less!