Slideshow: Long captions cut off on mobile

When a slideshow is added to a page, and the images have long captions (greater than 47 characters), the captions are cut off on the mobile view.

#### Steps to reproduce the issue

1. Insert a gallery into a post or page.
2. Add captions to the gallery images that are longer than 47 characters. Example:

> Marilyn Monroe helps kick off the construction of the Time & Life Building (July 2, 1957).
3. Publish the Page.
4. View it on mobile.

#### What I expected
To see the full text of the caption on top of the slideshow images or below them.

#### What happened instead
The text gets cut off without notation that there's more around 47 characters.


Reproduced on https://test3737373.com/gallery-2/
Reported in #3090609-t

Begin by reproducing the issue at the linked gallery URL using the listed steps and a caption longer than 47 characters on a mobile view. Trace the slideshow implementation responsible for rendering captions, then verify that the complete caption is visible above or below the image without being silently cut off.

Written by the indexing model from the issue text.