feat: Add direct ICS export URL

[iamareebjamal]

For fossasia/open-event-frontend#5072

[codecov]

Codecov Report

Merging #7414 into development will increase coverage by 0.04%.
The diff coverage is 82.81%.

@@               Coverage Diff               @@
##           development    #7414      +/-   ##
===============================================
+ Coverage        64.70%   64.75%   +0.04%     
===============================================
  Files              263      265       +2     
  Lines            13284    13313      +29     
===============================================
+ Hits              8596     8621      +25     
- Misses            4688     4692       +4     

Impacted Files	Coverage Δ
app/api/helpers/permissions.py	36.20% <0.00%> (+0.30%)	⬆️
app/api/helpers/tasks.py	16.75% <ø> (ø)
app/api/custom/calendars.py	50.00% <50.00%> (ø)
app/api/helpers/calendar/ical.py	97.29% <97.29%> (ø)
app/api/helpers/ICalExporter.py	100.00% <100.00%> (+2.85%)	⬆️
app/instance.py	88.04% <100.00%> (+0.13%)	⬆️
app/models/event.py	78.63% <100.00%> (+1.90%)	⬆️

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[lgtm-com]

This pull request fixes 1 alert when merging 6e4fb73 into 41c0115 - view on LGTM.com

fixed alerts:

• 1 for Module is imported with 'import' and 'import from'