Designs: return an error if uploading designs with duplicate names

What does this MR do?

This MR adds a fix for #235227 (closed) and a partial fix for #219113 (closed). See #219113 (comment 354495098)

Now, SaveDesignsService will validate that files passed to it have unique file names and produce an error response if not #219113 (comment 354825433) This behavior will save us from hitting the PG::UniqueViolation, as now it won't try to write duplicate entries in the database.

This MR only fixes the following two issues:

1. Upload a single testing.jpg file first, and then attempt to upload two different testing.jpg files, we get the following error:

PG::UniqueViolation

ERROR:  duplicate key value violates unique constraint "design_management_designs_versions_uniqueness"
DETAIL:  Key (design_id, version_id)=(7505, 6712) already exists.

2. When there are no existing testing.jpg designs uploaded, and I try to upload two testing.jpg files, I get:

Gitlab::Git::Index::IndexError (A file with this name already exists):

But there are other issues as well related to the uploading of designs (mentioned by @.luke here #219113 (comment 354825433)) which we need to take care of (maybe after this MR):

By @luke :

• Even after we decouple design identity with its filename (#213782 (closed) and #213783 (closed)) I think we would still have this problem when creating files.

• We should also handle when original_filenames are stripped of special characters #219113 (comment 360806168)

Screenshots

Does this MR meet the acceptance criteria?

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