fix: event invoice to be accessed by identifier

[shreyanshdwivedi]

Fixes #6291

Short description of what this resolves:

Currently, we can find event invoice only via event-invoices/int:id but it should be allowed to be fetched by identifier too - event-invoices/ just like for orders

Changes proposed in this pull request:

• adds event_invoice_identifier to routes and make proper changes to accommodate it

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[shreyanshdwivedi]

@iamareebjamal @prateekj117 made the changes. The routes now support both id and identifiers. Please have a look

[shreyanshdwivedi]

AFK. Will fix travis by tonight

[codecov]

Codecov Report

Merging #6292 into development will decrease coverage by 0.12%.
The diff coverage is 14.28%.

@@               Coverage Diff               @@
##           development    #6292      +/-   ##
===============================================
- Coverage        65.45%   65.32%   -0.13%     
===============================================
  Files              287      287              
  Lines            14661    14695      +34     
===============================================
+ Hits              9596     9600       +4     
- Misses            5065     5095      +30

Impacted Files	Coverage Δ
app/api/users.py	30.23% <0%> (-0.91%)	⬇️
app/api/discount_codes.py	22.27% <0%> (-0.5%)	⬇️
app/api/events.py	21.89% <0%> (-0.27%)	⬇️
app/api/__init__.py	100% <100%> (ø)	⬆️
app/api/helpers/permission_manager.py	28.01% <14.28%> (-0.35%)	⬇️
app/api/event_invoices.py	64.81% <25%> (-11.38%)	⬇️

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[shreyanshdwivedi]

@iamareebjamal fixed travis. Please have a look