Settings to control when the "alt" badge appears on images

[nikclayton]

Is your feature request related to a problem? Please describe.

Currently the "alt" badge appears on images that have alternative text (i.e., a description).

Since this obscures some of the image it's effectively "penalising" the poster for including the alt text. And as posters become more consistent about posting with alt text more and more images will display this badge.

Describe the solution you'd like

3-state enum to represent the user's preference for displaying the alt-text badge, values are:

1. Never
2. The description is missing
3. The description is present

The default value for new installs would be 2. The default value for users upgrading would be 3, to match the current behaviour.

The badge should probably be different depending on the setting as well. The existing "ALT" text can probably be retained, but if it's being shown because the text is present it should use a container colour and include a "tick" symbol, and if it's being shown because the text is not present it should use the error container and a "cross" symbol.