Provide self-registering storage system

[zeripath]

This PR represents a continuation of some work on #12813.

It provides for a self-registering storage solution and means that we can (mostly) avoid changing code outside of modules/storage if we add a storage system.

I think there is an extension to this to do with the default mapping and further improvements to do with default settings but this gives the first steps for this.

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-commenter]

Codecov Report

Merging #12978 into master will decrease coverage by 0.00%.
The diff coverage is 46.92%.

@@            Coverage Diff             @@
##           master   #12978      +/-   ##
==========================================
- Coverage   42.62%   42.61%   -0.01%     
==========================================
  Files         672      673       +1     
  Lines       73785    73797      +12     
==========================================
+ Hits        31448    31452       +4     
+ Misses      37247    37244       -3     
- Partials     5090     5101      +11     

Impacted Files	Coverage Δ
cmd/migrate_storage.go	0.00% <0.00%> (ø)
models/unit_tests.go	73.83% <ø> (-0.49%)	⬇️
modules/setting/setting.go	47.50% <ø> (-0.12%)	⬇️
modules/storage/minio.go	2.04% <13.33%> (+2.04%)	⬆️
modules/storage/local.go	39.62% <50.00%> (+1.52%)	⬆️
modules/storage/storage.go	46.87% <50.00%> (+8.41%)	⬆️
modules/storage/helper.go	54.54% <54.54%> (ø)
modules/setting/storage.go	72.97% <71.42%> (+29.49%)	⬆️
modules/setting/attachment.go	100.00% <100.00%> (+35.71%)	⬆️
modules/setting/lfs.go	43.90% <100.00%> (-1.10%)	⬇️
... and 12 more

---

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[lafriks]

Please resolve conflicts ;)

[zeripath]

conflicts resolved

[lafriks]

@lunny please review

[codecov-io]

Codecov Report

Merging #12978 into master will increase coverage by 0.10%.
The diff coverage is 46.61%.

@@            Coverage Diff             @@
##           master   #12978      +/-   ##
==========================================
+ Coverage   42.25%   42.35%   +0.10%     
==========================================
  Files         675      676       +1     
  Lines       74520    74532      +12     
==========================================
+ Hits        31485    31569      +84     
+ Misses      37922    37826      -96     
- Partials     5113     5137      +24     

Impacted Files	Coverage Δ
cmd/migrate_storage.go	0.00% <0.00%> (ø)
models/migrations/migrations.go	2.44% <0.00%> (ø)
models/unit_tests.go	73.83% <ø> (-0.49%)	⬇️
modules/setting/setting.go	46.13% <ø> (-0.11%)	⬇️
modules/storage/minio.go	2.04% <13.33%> (+2.04%)	⬆️
modules/context/api.go	75.22% <50.00%> (ø)
modules/storage/local.go	39.62% <50.00%> (+1.52%)	⬆️
modules/storage/storage.go	46.87% <50.00%> (+8.41%)	⬆️
modules/storage/helper.go	54.54% <54.54%> (ø)
modules/setting/storage.go	72.97% <71.42%> (+29.49%)	⬆️
... and 19 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[6543]

failed unrelated