Automatically seed Experience CS Projects (#757)

This is a small quality-of-life update for Experience CS developers to
automatically invoke the rake task
'projects:create_experience_cs_examples' when editor-api is seeded.

This will save excs developers having to (a) know and (b) remember to
invoke this task when resetting the database.

## Status

- Closes _add issue numbers or delete_
- Related to _add issue numbers or delete_

## Points for consideration:

- Security
- Performance

## What's changed?

- Adds `projects:create_experience_cs_examples` to the list of rake
tasks invoked automaticaly in `seeds.rb`.

## Steps to perform after deploying to production

None. Development-only change.

Author: fspeirs

db/seeds.rb

@@ -5,4 +5,5 @@
 if Rails.env.development?
   Rake::Task['projects:create_all'].invoke
   Rake::Task['for_education:seed_a_school_with_lessons_and_students'].invoke
+  Rake::Task['projects:create_experience_cs_examples'].invoke
 end