refactor: implement database module and abstract backend

[aymanbagabas]

• Separate database sql logic from backend
• Add database migrations support
• Add Postgres support
• Add database models
• Make the backend module concrete
• Tidy modules and clean up unused code

[codecov]

Codecov Report

Merging #337 (f339f2b) into main (7cd0583) will increase coverage by 4.37%.
The diff coverage is 60.69%.

@@            Coverage Diff             @@
##             main     #337      +/-   ##
==========================================
+ Coverage   46.33%   50.70%   +4.37%     
==========================================
  Files          88      107      +19     
  Lines        7753     8179     +426     
==========================================
+ Hits         3592     4147     +555     
+ Misses       3879     3746     -133     
- Partials      282      286       +4     

Impacted Files	Coverage Δ
git/commit.go	25.00% <ø> (ø)
git/patch.go	56.33% <0.00%> (+56.33%)	⬆️
server/access/access.go	91.66% <ø> (ø)
server/backend/hooks.go	0.00% <0.00%> (ø)
server/backend/utils.go	0.00% <0.00%> (ø)
server/config/file.go	100.00% <ø> (ø)
server/stats/stats.go	90.90% <ø> (ø)
server/sync/workqueue.go	50.00% <ø> (ø)
server/ui/common/utils.go	0.00% <0.00%> (ø)
server/ui/components/code/code.go	25.16% <0.00%> (+0.32%)	⬆️
... and 78 more

... and 1 file with indirect coverage changes