Unite green buttons under one css class

What does this MR do?

Unite green buttons under one css class. Rename btn-new, btn-create and btn-save to btn-success (which is default Bootstrap class).

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42325#note_56961473

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides