This change adds two post-migrations that enforce a character limit
for design_management_designs.filename of 255.

The first post-migration adds the limit to the filename column.

The second post-migration modifies any design filenames larger than 255
characters to the string "gitlab-modified-#{id}".

This will ensure the filename remains valid (by having a valid image
extension .jpg and being unique).

Any records modified by this data migration will appear as a broken
image when viewed within the GitLab app, as filenames are used to look
up the design file from the design git repository.

However, this is an intentional consequence given the 255 character
limit was selected because that is the largest filename that most
filesystems support, and also because GitLab.com has 6 records with
filenames larger than this but they are all abuse.

https://gitlab.com/gitlab-org/gitlab/-/issues/209025