完善 MySQL 账号管理，支持新增、授权、改密和删除

[hhyo]

相关issue：#23

• 支持 MySQL 账号创建、授权、改密和删除，暂不支持取消授权和ALL权限授权操作
• 使用django-mirage-field对用户名和密码数据进行加密
• 优化一个会话管理的风险点，不执行前端传来的SQL，由后端自行构建
• 账号名和密码校验规则待完善
• 测试用例待完善

[codecov]

Codecov Report

Merging #452 into master will decrease coverage by 1.35%.
The diff coverage is 16.44%.

@@            Coverage Diff             @@
##           master     #452      +/-   ##
==========================================
- Coverage    84.6%   83.24%   -1.36%     
==========================================
  Files          71       72       +1     
  Lines        9192     9383     +191     
==========================================
+ Hits         7777     7811      +34     
- Misses       1415     1572     +157

Impacted Files	Coverage Δ
sql/db_diagnostic.py	11.42% <0%> (-0.58%)	⬇️
sql/instance.py	46.19% <0%> (+3.39%)	⬆️
sql/models.py	95.12% <100%> (+0.13%)	⬆️
sql/urls.py	100% <100%> (ø)	⬆️
sql/views.py	46.19% <50%> (ø)	⬆️
sql/instance_account.py	7.65% <7.65%> (ø)
sql/admin.py	98.26% <87.5%> (-0.81%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1ff6a77...d0c3c2a. Read the comment docs.