Event model: Add info about presence of sessions,speakers

[abhinavk96]

Fixes #5529

Short description of what this resolves:

Eliminates the need to fetch all sessions and speakers of an event just to check if they exist.

Changes proposed in this pull request:

• Add field has_sessions to the event model
• Add field has_speakers to the event model

[codecov]

Codecov Report

Merging #5560 into development will increase coverage by <.01%.
The diff coverage is 75%.

@@               Coverage Diff               @@
##           development    #5560      +/-   ##
===============================================
+ Coverage        63.25%   63.26%   +<.01%     
===============================================
  Files              265      265              
  Lines            12726    12734       +8     
===============================================
+ Hits              8050     8056       +6     
- Misses            4676     4678       +2

Impacted Files	Coverage Δ
app/api/schema/events.py	90.07% <100%> (+0.14%)	⬆️
app/models/event.py	81.3% <66.66%> (-0.37%)	⬇️

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[iamareebjamal]

@CosmicCoder96 This will slow down the list queries by a lot. This should be handled by a separate relationship to event

[iamareebjamal]

As mentioned, this slowed down queries by large amount