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

Allow broadcast messages to be edited

Closes #3046 

See merge request !2268

Update button style on milestone pages

See merge request !2414

Update documentation to Go 1.5.3

Security fix: https://groups.google.com/forum/#!topic/golang-dev/MEATuOi_ei4

See merge request !2418

Fix has been introduced in !2224.

Move doc_styleguide in the development directory

Reference #3841 

And also add many new styleguides:

- Naming
- Images
- Links
- Headings
- Formatting
- Notes
- API

See merge request !2386

Added hint that you can search users by name, username, or email.

Fixes gitlab-org/gitlab-ee#19

/cc @JobV 

See merge request !2411

- Fix some syntax/grammar typos
- Link to GFM documentation on newlines
- Be less strict on the alphabetical order styleguide
- You can override the "numbers in headings" rule if you discuss it first
- Do not mention CE in notes if the feature is in both CE and EE

[ci skip]

Closes #3046

- 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

- Adds default values for `color` and `font` attributes
- Adds `active?`, `started?`, `ended?`, and 'status' methods

Now it returns the fully-formatted message so we can be consistent about
how it's shown.

Also make the feature tests less brittle.

Also resets the default example message when the user input is blank.

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

This is moved to a model method rather than an helper method because the
API will need it too.

See merge request !2406

Fix #9963 reference_filter "Encoding::CompatibilityError" bug with some complex URL;

https://github.com/gitlabhq/gitlabhq/pull/9964

@DouweM

See merge request !2383

Extend projects API with CI data

Reference #4264 

See merge request !2303

enable milestone filter to stick



See merge request !2241

Sampling data at a fixed interval means we can potentially miss data
from events occurring between sampling intervals. For example, say we
sample data every 15 seconds but Unicorn workers get killed after 10
seconds. In this particular case it's possible to miss interesting data
as the sampler will never get to actually submitting data.

To work around this (at least for the most part) the sampling interval
is randomized as following:

1. Take the user specified sampling interval (15 seconds by default)
2. Divide it by 2 (referred to as "half" below)
3. Generate a range (using a step of 0.1) from -"half" to "half"
4. Every time the sampler goes to sleep we'll grab the user provided
   interval and add a randomly chosen "adjustment" to it while making
   sure we don't pick the same value twice in a row.

For a specified timeout of 15 this means the actual intervals can be
anywhere between 7.5 and 22.5, but never can the same interval be used
twice in a row.

The rationale behind this change is that on dev.gitlab.org I'm sometimes
seeing certain Gitlab::Git/Rugged objects being retained, but only for a
few minutes every 24 hours. Knowing the code of Gitlab and how much
memory it uses/leaks I suspect we're missing data due to workers getting
terminated before the sampler can write its data to InfluxDB.