Add ability to import calendar events from an iCalendar file

[raulcodes]

Proposed change

This adds the ability to import an iCalendar file and add its events to an existing local calendar.
Addresses the feature requested here

This PR adds an 'upload' icon button to the top toolbar of the Calendar panel. Once clicked, a dialog opens that allows a user to upload an iCalendar (.ics) file and select from a list of existing calendars. Once submitted, the events of the iCalendar file are added to the given calendar.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion: discussion
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io