Show referenced MRs & Issues only when the current viewer can access them

This addresses both issues identified in #6066.

## The private MR by user `remy2` with a note referencing to a public issue

![Screen_Shot_2016-01-12_at_16.45.02](/uploads/c245ec2c1fdea1f9ba05183c24e142d9/Screen_Shot_2016-01-12_at_16.45.02.png)

---

## The public issue viewed by user `remy` **who doesn't have access to `remy2/private-project`** before the fix

![Screen_Shot_2016-01-12_at_18.14.50](/uploads/8db5580e803f5bddd6cb935233c579a0/Screen_Shot_2016-01-12_at_18.14.50.png)

---

## The public issue viewed by user `remy` **who doesn't have access to `remy2/private-project`** with the fix

![Screen_Shot_2016-01-13_at_12.02.32](/uploads/cb199f7b78191fba486a11412412e307/Screen_Shot_2016-01-13_at_12.02.32.png)

---

## The public issue viewed by user `remy2` with the fix (no change)

![Screen_Shot_2016-01-13_at_11.54.06](/uploads/ddece590d69f597a95559beddcd36660/Screen_Shot_2016-01-13_at_11.54.06.png)


See merge request !2405

- Removes redundant "Create a broadcast message" scenario that was
  entirely covered by the "Create a customized broadcast message"
  scenario.
- Adds "Edit an existing broadcast message" scenario
- Adds "Remove an existing broadcast message" scenario

Also make the feature tests less brittle.

- Improve specs for private Issue/MR referenced in public Issue
- Add specs for private Issue/MR referenced in public MR

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Show 'New Merge Request' buttons on canonical repo. (by @joshfng)

Replaces https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2189 to run tests on our runners.

See merge request !2300