Form to make Git PUSH from the dashboard

Related to #2.

We should provide the developer with a form where:

• user can push the committed changes to remote
• choose to trigger a ci build or not (`git push -o ci.skip`)
• choose variables that will be used as CI variables for the build that is automatically triggered after push

More details about PUSH options can be found here: https://docs.gitlab.com/ee/user/project/push_options.html

This form is going to be very close to the Trigger build form (#7 ). So, we should re-use components as much as we can.