fix(css):center images

[jvoisin]

Before:

After:

[fguillot]

This is more a matter of personal preference. It's probably best to include it in your custom stylesheet.

[jvoisin]

Is it? I can't think of a single website with left-aligned pictures :/

[Hyask]

As a long time Miniflux user, I indeed always wondered why the pictures wouldn't be centered.

[magne4000]

I would also expect images to be centered by default, it looks cleaner.

[fguillot]

Everything in Miniflux is left-aligned: entry images, the HTML5 media player, YouTube videos, image attachments, form settings, and so on.

I get complaints every single time something changes in the web UI :)

If I merge this PR, someone will inevitably complain that the images in the article body are no longer aligned with those in the enclosure list.

[jvoisin]

Fair, but this isn't really about the UI, and more about the content :P

Happy to send a bigger pull-request to make all the feed content centered :)