Create image selection dialog for entity push forms

Problem/Motivation

Mastodon allows for a image to be attached to the status when pushing status updates. The mastodon api entity module does not take this into account and it is currently not possible to attach an image to a toot in the mastodon api entity push form.

Proposed resolution

There are a few things to consider here.
- The mastodon api allows for multiple images to be attached. The module has the ability to do this via the API class.
- Alt text should be supplied for all images.
- Images should be passed through an image filter to try and reduce their size before transmitting.
- Different entities have different fields and so different types.

We would need to change the mastodon api entity form to allow an image field to be selected from the selected list of entity bundles.

Multiple entity bundles can be selected so there might be the possibility of bundles not having the correct field in place. Add an additional field for the image format to be selected for the image.

The mastodon api entity push form should then be updated to show the images (preferably as thumbnails or smaller images) and a text box for each to edit the alt text for each image. If the field doesn't exist for the entity then don't

Remaining tasks

None.

User interface changes

Addition of a selection form to the mastodon api entity form, along with an image style, which would then show the images and alt text on the push forms.

API changes

None.

Data model changes

The mastodon api entity configuration needs to be updated to include information about image fields selected. This won't effect all existing entities.