Add hidden caption

Problem/Motivation

We needed a second caption that is visually hidden to provide more details to people using screen readers.

User interface changes

Adds an extra setting and extra text area.

Data model changes

Adds fields to the table.

I am not expecting this to get added to the module as I imagine it's rather an edge case but, since I had to build it for the client, I figured I would share it for whomever might find it useful.

Some caveats:

• I changed the schema but there's no matching update function because we are installing the module fresh.
• I didn't do anything with the views integration because we don't need it.
• I don't understand image_field_caption_field_diff_view_alter() enough to know what I need to do there so there's just a "todo".
• You need to have .visually-hidden set to display: none in your theme or the hidden caption will show.
• The captions show one right after another so that may end up being confusing to screen readers unless you are careful with how they are worded. This may get refined at some point after we see how it works in practice.