Remet en place le lien d'abonnement au calendrier des congés

[florimondmanca]

Motivation

Je l'avais retiré par erreur dans #437

Ce lien permet d'ajouter le calendrier au format iCal dans son application de calendrier (Thunderbird, Infomaniak Calendar, etc)

Aperçu

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.37%. Comparing base (0daa72d) to head (49bdd6e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #445   +/-   ##
=======================================
  Coverage   90.37%   90.37%           
=======================================
  Files         191      191           
  Lines        2254     2254           
  Branches      165      165           
=======================================
  Hits         2037     2037           
+ Misses        217      208    -9     
- Partials        0        9    +9     

Flag	Coverage Δ
	90.37% <ø> (?)

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