Split up rake db:seed and rake db:seeds:fetch

rake db:seeds:fetch can be invoked by hand now, and rake db:seed no
longer automatically calls it. This allows local edits to seeds.sql

When seeding database, do not drop and recreate the local database
according to an online schema - instead keep the existing local
database. This prevents changes from local migrations from being
overwritten when seeding the database
Fixed language path for languages with non-URI chars
Merge pull request #2071 from bluerogue251/allow_local_changes_to_db_for_seeds

Allow local changes to db seeds
Merge pull request #2073 from ErikSchierboom/languageuri

Fixed language path for languages with non-URI chars
Fix markup of comments

Fixes #2072
Merge pull request #2074 from jalyna/fix-mentions

Fix markup of comments
Update state of all submissions when done

When a user had one or more submissions whose state was not
`superseded` and that user decided to post a new submission and close
that exercise. Then the latest submissions state would correctly get
changed to `done`, whereas all the old submisssions would not have their
states updated to `superseded`. This has now been fixed and updated to
correctly change the state of old submissions.

* Add test that state of previous submission changes
* Update completion to change state of old submissions
* Update completion_test to assert that old submissions states are updated
* Remove unused variable in completion_test