Add `fallback` image style

Problem/Motivation

We need a default fallback image style to protect a website from users uploading large image files and having those files rendered to the end-users.

Steps to reproduce

1. Create a `Page` content and upload a 50 MB image to all image fields.
2. Save the page and observe that Banner image (and other images) are shown using their original file (and size).

Proposed resolution

Add a default `fallback` image style that will be applied to all images that do not specify a custom image style explicitly.

The pre-processing for images currently takes place within `civictheme_media_image_get_variables()` function. This function accepts an `$image_style` parameter which is currently set to `NULL`. The proposal is to create a `civictheme_fallback` image style with some sensible defaults (downscale to 1500x1500, preserving ratio) and set this as a default value for the `$image_style` parameter. This will ensure that all images pre-processed via CivicTheme's functions will be downsized.

Note that consumer sites can always update the proposed default values of the `fallback` image style as they see fit.

Remaining tasks

1. Decide on the image style name. Proposal `civictheme_fallback`.
2. Decide on the default values. Proposal is `1500x1500`
3. Add image style
4. Add image style name to the `civictheme_media_image_get_variables()` function as a default value for the second argument.
5. Add tests that assert the present of the `styles` substring in the rendered output of, say, Banner featured image source URL.

User interface changes

None

API changes

None

Data model changes

None