FEATURE: Add event location/description and "My Events" filter

[SamSaffron]

This pull request introduces two major new features to the calendar plugin: the ability to add a location to an event, and a new "My Events" view on the upcoming events page.

Event Location

You can now add a location to an event. This is a free-text field that can be used for a physical address, a URL, or any other location details.

• A location field has been added to the event builder modal.
• The location is displayed in the event details in the post, complete with a new "location-pin" icon. URLs within the location field are automatically linked.
• This is supported by a database migration to add the location column, and updates to the event model, serializer, and parser.

Event description

You can now add a description to an event. This is a free-text field that can be used to describe your event.

• A description field has been added to the event builder modal.
• The description is displayed in the event details in the post, complete with a new "circle-info" icon. URLs within the location field are automatically linked.
• This is supported by a database migration to add the description column, and updates to the event model, serializer, and parser.

"My Events" Filter

The /upcoming-events page now includes tabs to switch between "All events" and "My events".

• The "My events" tab shows all upcoming events that the current user is "Going" to.
• This creates a personalized calendar for users to easily see their own upcoming schedule.
• A new /upcoming-events/mine route has been added, and the backend event finder now supports filtering by an attending_user.

Other Improvements

• The calendar view on the /upcoming-events page now defaults to a "list" view on mobile for a better experience.
• The "Open Event" and "Close Event" actions now have a disabled/saving state to provide better feedback during the operation.
• System tests have been added to cover the new functionality.