Make single diff patch limit configurable

What does this MR do?

• Creates a new column to hold the single patch limit value on application_settings
• Allows updating this value through the application_settings API
• Calculates single diff patch collapsing limit based on diff_max_patch_bytes column
• Updates diff limit documentation
• Adds documentation (with warning) as of how one can update this limit

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48027

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides