Fix typo: calender → calendar

[waldyrious]

Fixes #9107

Short description of what this resolves:

Just a small refactoring to correct a misspelling of "calendar".

Changes proposed in this pull request:

The following changes have been made:

• Files add-to-calender.(ts|hbs) were renamed to add-to-calendar.(ts|hbs)
• Files add-to-calender-join-video.(ts|hbs) were renamed to add-to-calendar-join-video.(ts|hbs)
• All references to the add-to-calender.hbs file in the *.po translation files were corrected to the new name
• The CSS classes including "calender" in the name were renamed to say "calendar" instead
• The TypeScript class AddToCalender was renamed to AddToCalendar
• The Handlebars components AddToCalender and AddToCalendarJoinVideo were renamed to say "Calendar" instead

Checklist

• [x] I have read the Contribution & Best practices Guide.
• [x] My branch is up-to-date with the Upstream development branch.
• [ ] The acceptance, integration, unit tests and linter 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)

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Name	Status	Preview	Comments	Updated (UTC)
open-event-frontend	❌ Failed (Inspect)			Apr 13, 2024 0:24am

[mariobehling]

Please resolve conflicts.

[waldyrious]

Please resolve conflicts.

Done :)