Improve GDK to make it easy to use with external IDEs

See original motivation and proposal here: #368

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Improve GDK to make it easy to use with external IDEs

Renamed:

• gdk run db => gdk run tests

New:

• gdk run dev
• gdk run dev_all
• gdk run jobs

Modified:

• gdk run app (no longer runs gitaly or background jobs)
• gdk run <inexistent> will fail with a message

This is also aware of rails5 new command.

You can gdk run app rails5 and gdk run jobs rails5

Closes #368