Report to detect duplicate file entries

A new report has been created that detects duplicate files in the file_managed table and suggests actions to take. Now, the report lists the following information for each set of duplicate files:

• File URI: The location of the file on the Drupal system.
• File name: Extracted from the URI.
• Main FID: The file identifier that is in active use.
• Unused FIDs: Identifiers of duplicate files without reference in file_usage.
• Recommendation: A suggestion based on the status of the duplicate files.

Scenarios

1. If there is a main file in use and unused duplicates: It is recommended to keep the main file and delete the unused duplicates.
2. If all duplicate files are unused: It is recommended to remove all duplicates as none are being used.
3. If all duplicate files have active usage: A manual review is required.

Also, I guess that process can be done directly using SQL. I mean writing a SQL query that returns what the values calculated in $main_fid and $unused_fids. Probably, this is not trivial, but if done will solve many performance problemas with sites that have huge number of files.

Optimized data handling and split the logic for displaying recommendation messages.

It looks good to me, but I'll wait for @lpeidro review.

Thanks!