Disable reference generation in preformatted/code blocks
### Summary
If a user adds text in code or preformatted text via Markdown or HTML that contains `#XXX`, the system adds a note that issue `XXX` was mentioned. This is particularly annoying because we often list gdb backtrace dumps into our issues, and many issues get mentioned as a result. For example:
```
(gdb) bt
#0 0x00000000004004c4 in second () at main.cc:6
#1 0x00000000004004d2 in first () at main.cc:11
#2 0x00000000004004dd in main () at main.cc:17
(gdb)
```
### Steps to reproduce
1. In an issue, write the above text using Markdown or HTML tags (e.g. `<code>`, `<pre>`).
2. Observe that [issue 1](https://gitlab.com/gitlab-org/gitlab-ce/issues/1) and [issue 2](https://gitlab.com/gitlab-org/gitlab-ce/issues/2) have a note that says they were mentioned.
### Expected behavior
Everything enclosed in the code blocks should be ignored as references.
### Observed behavior
Issues get referenced unnecessarily.
### Fix
I've made `reference_extractor.rb` strip out HTML and Markdown blocks before processing. I considered running the raw text through the entire Markdown processor, but this seems overkill and perhaps could lead to some unintended side effects.
See merge request !365
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.
