add .gpg url (match github behaviour)

[sapk]

After some testing of #4193 I added what was missing to have it working. Thx to @techknowlogick that lay down most of the bases.

Basically, we haven't in database what is needed to fully reconstruct the gpg armored export (ex: signature, full identities, ...).
I add a table containing the imported armored key that we can re-used to be join with others imported keys and exported.
Like github previous keys that need re-import are marked as a header (ex: https://github.com/sapk.gpg)

The only bug is that the cipherTypeByte are changed by the export by golang compared to the imported one but that doesn't change the interpretation of the gpg armored key.

[techknowlogick]

@sapk Thanks for continuing this PR 😃

[sapk]

For info, imported vs exported key

[codecov-io]

Codecov Report

Merging #6610 into master will increase coverage by <.01%.
The diff coverage is 43.11%.

@@            Coverage Diff             @@
##           master    #6610      +/-   ##
==========================================
+ Coverage   40.47%   40.48%   +<.01%     
==========================================
  Files         405      406       +1     
  Lines       54388    54484      +96     
==========================================
+ Hits        22012    22056      +44     
- Misses      29349    29390      +41     
- Partials     3027     3038      +11

Impacted Files	Coverage Δ
models/migrations/migrations.go	1.52% <ø> (ø)	⬆️
models/user.go	49.76% <ø> (ø)	⬆️
models/error.go	34.06% <0%> (-0.42%)	⬇️
models/migrations/v84.go	0% <0%> (ø)
models/models.go	56.59% <100%> (+0.18%)	⬆️
models/gpg_key.go	53.37% <39.62%> (-1.87%)	⬇️
routers/user/home.go	37.63% <46.87%> (+0.7%)	⬆️
routers/user/profile.go	35.77% <90.9%> (+2.44%)	⬆️
modules/log/event.go	65.98% <0%> (+1.52%)	⬆️

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[lunny]

@sapk @techknowlogick good work. With integration tests are better.

[sapk]

@lunny done