Add delayed_job gem to handle resample.

[an-ju]

• Add delayed_job gem.
• Handle resample asynchronously by delayed_job worker.
• Add a resample method to projects.
• Update projects index page.

I added delayed_job gem to handle asynchronous jobs. Including this gem will provide necessary infrastructure to handle more complex functionalities (such as grading as I'm about to work on). Adding this gem requires enabling delayed_job worker on heroku side, which is free to use.

I added a resample button in the index page so that TAs can get instant feedback.

There is no test.

[tansaku]

@an_ju - you might also want to consider using the heroku scheduler that allows us to manage hourly and daily tasks - simply create the rake task for what you need.

I'm not inclined to merge code that doesn't have tests. We need a specific feature describing what things are for, with acceptance tests to drive that feature before we can justify adding code to the project.

My suspicion is that we don't actually need this and that plain heroku scheduling can handle everything you need. If it can't let's hear the specific user stories - thanks

[an-ju]

Right. After another talk with Armando, I think this feature doesn't need to be added to ProjectScope right now.

The story was to enable real-time resampling, which might take a very long time so should be run in background.

I'll close this PR. Thanks for reviewing!