Ability to move issue to another project
Tasks:
- [x] Create scaffold of service that will move issue to another project.
- [x] Close old issue, add system note about moving issue to a new project.
- [x] Create a new issue, add system note about issue being moved from old project.
- [x] Check if issue can be moved to another project before executing service
- [x] Check permissions when moving an issue (`:admin_issue` ability)
- [x] Display select box for a new project when editing an issue
- [x] Show only projects that issue can be moved into in that select box
- [x] Add project select handler, helper and some permission filters to it
- [x] Preserve as much information as possible, including author
- [x] Prepare mechanisms that unfolds local references in issue description
- [x] Rewrite issue description with references unfolding and add some specs for it
- [x] Rewrite all system notes and comments attached to issue that is being moved
- [x] Update `Label` so that is was able to create cross reference labels (separate MR)
- [x] Add notifications about moving issue to another project
- [x] Display confirmation alert/message when issue move has been requested
- [x] Make it possible to undo selecting project where issue will be moved to
- [x] Add column to issue, that will indicate if it has been moved to another project
- [x] Do not allow to move issue that has been already moved
- [x] Write top-to-bottom feature spec in RSpec instead of Spinach
UI:


Closes#3024
See merge request !2831
Store both the project and identifier of extracted references. This
prevents `ReferenceExtractor` from returning objects in the wrong
project for cross-project references.
Any mention of Issues, MergeRequests, or Commits via GitLab-flavored markdown
references in descriptions, titles, or attached Notes creates a back-reference
Note that links to the original referencer. Furthermore, pushing commits with
commit messages that match a (configurable) regexp to a project's default
branch will close any issues mentioned by GFM in the matched closing phrase.
If accepting a merge request would close any Issues in this way, a banner is
appended to the merge request's main panel to indicate this.