Refactor load calculation

[samuelwei]

Fixes #233

Type (Highlight the corresponding type)

• Bugfix
• Feature
• Documentation
• Refactoring (e.g. Style updates, Test implementation, etc.)
• Other (please describe):

Checklist

• Code updated to current develop branch head
• Passes CI checks
• Is a part of an issue
• Tests added for the bugfix or newly implemented feature, describe below why if not
• Changelog is updated
• Documentation of code and features exists

Changes

• Refactor load calculation to be similar to Scalelite
• Add plugin system to implement custom load calculation

Other information

Before this PR the load was calculated based on the video streams, voice participants and participants.
After this PR the load is only calculated based on the amount of participants, however during the starting phase of a meeting the number of participants per meetings is set to min. amount. The duration of the starting phase and the min. amount can be changed using BBB_LOAD_MIN_USER_COUNT and BBB_LOAD_MIN_USER_INTERVAL. For meeting with a user limit defined, we use this during the starting phase if it is lower than the global config.

[codecov]

Codecov Report

Attention: Patch coverage is 96.87500% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 95.80%. Comparing base (b86c6cf) to head (7edeb7d).

Files	Patch %	Lines
...p/Plugins/Defaults/ServerLoadCalculationPlugin.php	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #956   +/-   ##
==========================================
  Coverage      95.80%   95.80%           
- Complexity      1074     1082    +8     
==========================================
  Files            176      178    +2     
  Lines           3815     3842   +27     
==========================================
+ Hits            3655     3681   +26     
- Misses           160      161    +1     

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