feat: subconversations store in core

[PatrykBuniX]

Description

There was some refactoring in the core package regarding subconversations and mls conference calls. Subconversations storage has been moved to core's indexedDB store.

For more details see ## Screenshots/Screencast (for UI changes)

Checklist

• PR has been self reviewed by the author;
• Hard-to-understand areas of the code have been commented;
• If it is a core feature, unit tests have been added;

[codecov]

Codecov Report

Merging #16131 (064d493) into dev (a929ff6) will increase coverage by 0.00%.
Report is 2 commits behind head on dev.
The diff coverage is 12.50%.

@@           Coverage Diff           @@
##              dev   #16131   +/-   ##
=======================================
  Coverage   44.54%   44.54%           
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  Files         708      708           
  Lines       23151    23155    +4     
  Branches     5259     5261    +2     
=======================================
+ Hits        10313    10315    +2     
- Misses      11498    11499    +1     
- Partials     1340     1341    +1