Add "remove source branch" checkbox when using create merge request in an issue

Description

The removal of the topic branch on a merge request merge is highly contextual and very hard to generalise as it is very policy-, workflow-, project-, or even issue-dependent. Yet the extremely convenient "Create Merge Request" in an issue does not offer the option and results in nasty surprises.

From https://gitlab.com/gitlab-org/gitlab-ce/issues/21143#note_49271376

Proposal

• Choose to remove or not remove when using the create merge request button in an issue.
• The default value is persisted according to the same logic as when using the dedicated full-blown Create Merge Request view.