Improve trix and action text support

[Paul-Bob]

Feature

Trix field have some limitations:

1. File attachment is possible only on persistent records Avo demo error: "You can't upload files into the Trix editor until you save the resource" #2713
2. File attachment is possible only whit explicit attachment_key
3. File attachments are corrupt except images on action text. (has_rich_text) After uploading File in ActionText (Trix) it can not be found when rendering (missing attachable) #2538

Let's explore this possibility:

1. When attachment_key is set that means we need a persistent record with that attachment_key association. So limitation points 1 and 2 still applying.
2. When attachment_key is not set that means the user do not want to associate the attachments to a fixed key/association. We can generate a random key like here https://trix-editor.org/js/attachments.js and in this case the limitation points 1 and 2 will not apply, i.e there is no need for a record and neither attachment_key.

[davekruse]

Editing an ActionText field with Trix field separates captions from images.

1. Add an image
2. Click and change the caption
3. Save
4. Edit

In the trix edit view, the caption is rendered as regular text (a couple
line breaks and plain text). Saving the field causes that caption to become part of the body. Rendering the ActionText elsewhere also shows the text as part of the body - no longer attached to the image.

[sedubois]

I think a helpful first step would be to ensure models using has_rich_text are added to the test suite and to the demo website. Currently the use of has_rich_text does not seem to be tested/demonstrated/documented anywhere. See issue.

I can also confirm that captions are broken as noted by @davekruse.