Refactor struct's time to remove unnecessary memory usage

[lunny]

This PR will remove unnecessary time.Time on almost all the structs and define a new time type TimeStamp int64 to handle the time. It's also improve the TimeSince where converted to time.Time and convert back before. So that, many BeforeInsert and AfterLoad is still unnecessary. The code is more clean.

[appleboy]

LGTM

[codecov-io]

Codecov Report

Merging #3142 into master will increase coverage by 0.1%.
The diff coverage is 63.1%.

@@            Coverage Diff            @@
##           master    #3142     +/-   ##
=========================================
+ Coverage   34.66%   34.77%   +0.1%     
=========================================
  Files         275      276      +1     
  Lines       40030    39945     -85     
=========================================
+ Hits        13878    13889     +11     
+ Misses      24173    24058    -115     
- Partials     1979     1998     +19

Impacted Files	Coverage Δ
modules/templates/helper.go	49.31% <ø> (-1.04%)	⬇️
models/login_source.go	8.45% <ø> (+0.07%)	⬆️
models/issue_watch.go	70.73% <ø> (+4.06%)	⬆️
models/notification.go	74.57% <ø> (-2.11%)	⬇️
models/repo_unit.go	45.83% <ø> (-2.17%)	⬇️
models/admin.go	81.63% <ø> (-0.73%)	⬇️
models/attachment.go	50% <ø> (-1.07%)	⬇️
models/branches.go	52.29% <ø> (-0.55%)	⬇️
models/webhook.go	67.47% <ø> (-0.18%)	⬇️
models/lfs.go	28.26% <ø> (+3.26%)	⬆️
... and 35 more

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[lafriks]

LGTM