refactor: custom-emoji functions out of meteor methods

[abhinavkrin]

Proposed changes (including videos or screenshots)

This pr refactors insertOrUpdateEmoji uploadEmojiCustom methods by extracting the logic to separate function and using theme in emoji-custom endpoint instead of the methods.

Refactoring the code will help moving forward with the fix for the update emoji issues.
Closes #26980
SUP-622
The goal is to break the PR #32768 into smaller prs.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.98%. Comparing base (5388eef) to head (3a5eba5).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #32925   +/-   ##
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  Coverage    54.98%   54.98%           
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  Files         2596     2596           
  Lines        55966    55966           
  Branches     11585    11585           
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  Hits         30771    30771           
  Misses       22552    22552           
  Partials      2643     2643           

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[MarcosSpessatto]

Can we add some API tests to cover this?

Also, can we rewrite the PR title to be clearer?

[abhinavkrin]

Can we add some API tests to cover this?

Also, can we rewrite the PR title to be clearer?

We already have tests for the emoji-custom endpoint. I will be adding tests along with my next pr for the fix.