Added URL mapping for Release attachments like on github.com

[al-sabr]

Implement a Git Hub style URL mapping for the attachment files in Release.

[al-sabr]

If this doesn't pass then I'm not giving anything more!

[techknowlogick]

@gdeverlant would you be able to resolve conflicts as well as update per @lafriks's suggestion?

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 months. Thank you for your contributions.

[codecov-io]

Codecov Report

Merging #1707 into master will decrease coverage by 0.01%.
The diff coverage is 4.34%.

@@            Coverage Diff             @@
##           master    #1707      +/-   ##
==========================================
- Coverage   37.82%   37.81%   -0.02%     
==========================================
  Files         322      322              
  Lines       47498    47544      +46     
==========================================
+ Hits        17966    17977      +11     
- Misses      26944    26982      +38     
+ Partials     2588     2585       -3

Impacted Files	Coverage Δ
routers/repo/repo.go	20.82% <0%> (-1.94%)	⬇️
models/attachment.go	43.62% <0%> (-3.48%)	⬇️
models/release.go	52.8% <0%> (-0.96%)	⬇️
routers/routes/routes.go	85.01% <100%> (+0.04%)	⬆️
models/repo_list.go	63.29% <0%> (-1.27%)	⬇️
models/repo_indexer.go	48.3% <0%> (+3.81%)	⬆️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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[rakshith-ravi]

Forgive me for bothering, but I'm just trying to understand: What exactly does this PR do? I'm not able to understand clearly. We already have releases with attachments, don't we?

[techknowlogick]

@rakshith-ravi it provides clean URLs for attachments (has filename in URL), instead of the URL being the UUID of the uploaded file.