Support configurable attachment size in Application Settings page
### What does this MR do?
This MR provides the ability to configure the maximum size of an attachment inside a note. A parameter has been added to the Application Settings page.
### Are there points in the code the reviewer needs to double check?
What should be done with the legacy note attachment validation? I added code to make the validation work with the configurable setting. I could see an issue where an admin lowers the limit from 10 megabytes to 5 megabytes, which could cause an existing model to be invalid.
### Why was this MR needed?
We often have attachments that exceed 10 MB, and it would be nice to be able to override the defaults.
### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?
See Issue #1258
### Screenshots
Before:
After:
See merge request !407
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
Douwe Maan
Robert Speicher
Dmitriy Zaporozhets
Dmitriy Zaporozhets
Jacob Vosmaer
Stan Hu
