Add repo_id for attachment

[lunny]

When create a new issue or comment and paste/upload an attachment/image, it will not assign an issue id before submit. So if user give up the creating, the attachments will lost key feature and become dirty content. We don't know if we need to delete the attachment even if the repository deleted.

This PR add a repo_id in attachment table so that even if a new upload attachment with no issue_id or release_id but should have repo_id. When deleting a repository, they could also be deleted.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@0767fe0). Click here to learn what that means.
The diff coverage is 52.85%.

@@           Coverage Diff           @@
##             main   #16958   +/-   ##
=======================================
  Coverage        ?   45.18%           
=======================================
  Files           ?      763           
  Lines           ?    86208           
  Branches        ?        0           
=======================================
  Hits            ?    38952           
  Misses          ?    40955           
  Partials        ?     6301           

Impacted Files	Coverage Δ
models/attachment.go	58.77% <ø> (ø)
routers/api/v1/repo/release_attachment.go	0.00% <0.00%> (ø)
routers/web/repo/setting.go	14.41% <0.00%> (ø)
services/wiki/wiki.go	57.07% <0.00%> (ø)
routers/web/repo/attachment.go	37.80% <42.85%> (ø)
models/repo.go	54.50% <50.00%> (ø)
services/attachment/attachment.go	75.00% <75.00%> (ø)
models/context.go	57.14% <100.00%> (ø)

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[6543]

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