New markdown feature: Mention a time

[timabbott]

We added a new markdown syntax, !time(2020-06-18T17:32:00-07:00), that is meant to be a timezone-aware time displayed to all clients in local time (using the same mechanisms used in the timestamps in Zulip messages).

There's likely some discussion that will happen on how these should be rendered; my current leaning is that we should put a clock icon/emoji at the start of the pills. But what's important for now is the core logic to make this feature work at all. In the webapp, it's this part of rendered_markdown.js:

    content.find('span.timestamp').each(function () {                                      
        // Populate each timestamp span with mentioned time                                
        // in user's local timezone.                                                       
        const timestamp = moment.unix($(this).attr('data-timestamp'));                     
        if (timestamp.isValid() && $(this).attr('data-timestamp') !== null) {              
            const text = $(this).text();                                                   
            const rendered_time = timerender.render_markdown_timestamp(timestamp,          
                                                                       null, text);        
            $(this).text(rendered_time.text);                                              
            $(this).attr('title', rendered_time.title);                                    
        } else {                                                                           
            $(this).removeClass('timestamp');                                              
            $(this).attr('title', 'Could not parse timestamp.');                           
        }                                                                                  
    });                                                                               

     // Renders the timestamp returned by the !time() markdown syntax.                          
exports.render_markdown_timestamp = function (time, now, text) {                           
    now = now || moment();                                                                 
    if (page_params.timezone) {                                                            
        now = now.tz(page_params.timezone);                                                
        time = time.tz(page_params.timezone);                                              
    }                                                                                      
    const timestring = time.format('ddd, MMM D YYYY, h:mm A');                             
    const titlestring = "This time is in your timezone. Original text was '" + text + "'."\
;                                                                                          
    return {                                                                               
        text: timestring,                                                                  
        title: titlestring,                                                                
    };                                                                                     
};                                                                                         

Implementing this should be a priority, since the whole point is that the time gets rendered properly :).

[timabbott]

The markup has changed to <time>, which will likely be easier to deal with.