feat: support to configure max database connections for site database users

[tanmoysrt]

Fixes #2315 #2317
(Check issue for more details)

• Clean up non required fields and functions of old implementation
• Limit database user's max connection. The total allowed connections for each site will be 16 by default. But upon any request, we can configure it from backend
• Frontend Changes
• Allow to set label/title for database user to identify easily (Fixes 2317)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 42.19%. Comparing base (f806bcb) to head (953586a).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
...s/doctype/site_database_user/site_database_user.py	0.00%	7 Missing ⚠️
press/press/doctype/agent_job/agent_job.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2316      +/-   ##
==========================================
+ Coverage   39.69%   42.19%   +2.49%     
==========================================
  Files         375      376       +1     
  Lines       28535    28691     +156     
==========================================
+ Hits        11328    12107     +779     
+ Misses      17207    16584     -623     

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