Add LastLogin to the User API

[zeripath]

This PR adds the LastLogin to the API returns for /api/v1/user /api/v1/users/:username and /api/v1/admin/users

[codecov-io]

Codecov Report

Merging #7196 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7196      +/-   ##
==========================================
- Coverage   41.52%   41.52%   -0.01%     
==========================================
  Files         449      449              
  Lines       61310    61314       +4     
==========================================
  Hits        25461    25461              
- Misses      32497    32501       +4     
  Partials     3352     3352

Impacted Files	Coverage Δ
modules/structs/user.go	100% <ø> (ø)	⬆️
models/user.go	50.41% <100%> (+0.09%)	⬆️
routers/api/v1/convert/convert.go	83.96% <100%> (+0.15%)	⬆️
routers/repo/view.go	42.23% <0%> (-1.02%)	⬇️

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[lunny]

will fix #7197

[sapk]

It maybe good to add at least small test to:

gitea/models/user_test.go

Line 26 in 8cd4c22

func TestUser_APIFormat(t *testing.T) {