Make it possible to edit certain fields of a past event

[psrpinto]

Before the event has started, the hosts can edit:

• Title
• Description
• Dates

After the event has started but no contributions:

• Title
• Description
• Dates

After the event has started but with contributions:

• Title
• Description
• Extend the end date

1 hour after the event has ended:

• Title
• Description
• Extend the end date

After one hour after the event has ended, they can edit:

• Description

Reflect updates by a message "This event was last updated x minutes ago", linking to a timeline.

[psrpinto]

Related #215 (comment)

[akirk]

@trymebytes and I discussed that it would be best to implement it by introducing even more custom capabilities, for example edit_translation_event_title:

<input
    type="text"
    id="event-title"
    name="event_title"
    value="<?php echo esc_html( $event_title ); ?>"
    required
    size="42"
    <?php echo current_user_can( 'edit_translation_event_title', $event_id ) ?: 'readonly'; ?>
>