feat: support featured images (resolves #281)

[cindyli]

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Description

Support the display of featured images on the grid of "Views" page and post articles.

Steps to test

1. Go to "Views" page;
2. Featured images are displayed in the grid and match the design; articles that don't have featured images display placeholder images;
3. Select an article that has the featured image;
4. The featured image should be displayed on the article page and match the design;
5. Go back to the "Views" page and select a post that doesn't have the featured image;
6. No images should be shown on the article page.

[cindyli]

@TeddTech , there was an issue that the post preview contents are narrower on the mobile layout. See the screenshot below. I fixed in the new commit. Can you take a look again? Thanks.