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:

![Screen_Shot_2015-03-29_at_3.06.53_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6013a1dbc8cf61a63e93744149937fa0/Screen_Shot_2015-03-29_at_3.06.53_PM.png)

After:

![Screen_Shot_2015-03-29_at_3.12.34_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/f3518af7e8653ba40f0a3579456da6ad/Screen_Shot_2015-03-29_at_3.12.34_PM.png)

See merge request !407