Add feature to skip queue/schedule

[MichaelKohler]

As of now, once a schedule to tweet is set, all tweets are only tweeted in those slots. This means that an "important" tweet can't skip the queue and can't be tweeted instantly.

Example:
Mozilla releases a new Firefox version at 18:00 CET. The configuration has a slot at 17:00 CET. The tweet won't be tweeted until 17:00 CET the next day.

User stories:

• As a tweet author I can set the schedule time to "now" so it will overwrite the pre-defined time slot and the tweet gets tweeted instantly once moved to the "to tweet" column
• As a reviewer I can remove the schedule of the tweet so it joins the normal queue again

Example of such an issue:

## Tweet should be about
Something awesome.

## Tweet Content
This is some tweet that needs to go out now, it should not wait to be tweeted!

## Scheduled for
now

Additionally, bonus points if the "string" that triggers this behavior can be configured in the config.

[freaktechnik]

• https://github.com/mozillach/gh-projects-content-queue/blob/master/lib/sources/tweeting.js#L88 has to be extended to add tweets with this special time set to the high priority queue (and maybe even unshift instead of push to make it the first candidate).
• https://github.com/mozillach/gh-projects-content-queue/blob/master/lib/tweet-card-content.js#L282 should not trigger for this special date. I think the nicest way of solving this would be to make ScheduledDate return the current TS whenever this special date format is set and have some flag to check if that format is set for the tweeting.js scheduler.

In a perfect world there would of course be tests for this. The one for the tweeting source may not be as simple, so is not necessary in my eyes.