Prepare uploaders for personal snippets comments

What does this MR do?

It adds support for uploading files that are not related to projects.

We have now 2 uploads controller:

• UploadsController - this controller handled only show files action and this functionality was not changed. I only added a create action for personal snippets and support for showing them.
• Projects::UploadsController

I created concern that is used by both of these controllers.

Why was this MR needed?

This is the last preparation step for enabling comments on personal snippets. When commenting personal snippets we should support uploading files to those comments as well. And that's why we need to have the possibility to do so.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#12910 (closed)