add generic webhook

[LeoQuote]

1. 重构 notify 部分的代码, 和engine 的类似, 也是提供一个 base Notifier, 属性有 workflow, audit, 都是原生的 django model, 开发者可以很方便地开发支持的通知方式
2. 增加 generic webhook 支持, 方便高度的定制需求.

fix #1480

[codecov]

Codecov Report

Attention: 43 lines in your changes are missing coverage. Please review.

Comparison is base (815cbd5) 75.50% compared to head (88cea36) 75.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2333      +/-   ##
==========================================
+ Coverage   75.50%   75.75%   +0.24%     
==========================================
  Files         107      108       +1     
  Lines       15460    15681     +221     
==========================================
+ Hits        11673    11879     +206     
- Misses       3787     3802      +15     

Files	Coverage Δ
archery/settings.py	58.92% <100.00%> (+0.37%)	⬆️
common/config.py	83.54% <100.00%> (+3.54%)	⬆️
common/tests.py	100.00% <ø> (ø)
sql/archiver.py	57.03% <100.00%> (+0.50%)	⬆️
sql/test_notify.py	100.00% <100.00%> (ø)
sql/tests.py	98.77% <100.00%> (-0.14%)	⬇️
sql/utils/execute_sql.py	81.35% <100.00%> (ø)
sql/utils/tests.py	100.00% <100.00%> (ø)
sql/utils/workflow_audit.py	97.01% <100.00%> (+0.02%)	⬆️
sql_api/serializers.py	79.88% <100.00%> (+0.11%)	⬆️
... and 5 more

... and 1 file with indirect coverage changes

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