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
317206300d
