Add setting that allows admins to choose which Git access protocols are enabled.

## What does this MR do?

It allows admins to disable one of the two protocols for Git access. They can choose to enable just SSH, HTTP or allow both. If one of them is disabled, the clone URL in the project will show only the allowed protocol, and no dropdown to change protocols will be presented.

## What are the relevant issue numbers?

Full implementation on GitLab's side for #18601 

GitLab Shell implementation: gitlab-org/gitlab-shell!62

GitLab Workhorse implementation: gitlab-org/gitlab-workhorse!51

## Screenshots (if relevant)

![Screen_Shot_2016-06-16_at_12.26.19_PM](/uploads/bad845142e9704a7385b2eaca51fd4eb/Screen_Shot_2016-06-16_at_12.26.19_PM.png)
![Screen_Shot_2016-06-20_at_4.24.54_PM](/uploads/6e452dd269e06f0be23841ce93866ed6/Screen_Shot_2016-06-20_at_4.24.54_PM.png)



/cc @jschatz1  this MR touches the UI. Please review.

See merge request !4696

Rename ENV['PROTOCOL'] to ENV['GL_PROTOCOL'] to conform to what GitLab Shell expects and make the `protocol` param in `GitAccess` mandatory.