feat: improve workshop preview images on the initiatives page (resolves #446)

[cindyli]

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Description

Applied changes described in #446 for the Initiatives page.

Steps to test

1. Go to Airtable -> base "WeCount_DEV", "preview_alt_text" and "cover_alt_text" fields should be in place;
2. Go to the initiatives page, check:

• Workshop preview images are no longer links;
• For workshops that have "preview_alt_text" set in the Airtable, alt value for this workshop preview image should have been set to the alt value;
• For workshops that don't have "preview_alt_text" set in the Airtable, this workshop preview image tags should have `role="presentation".

3. Click on a workshop title to go to the workshop information page. Check:

• If this workshop has a cover image uploaded in the Airtable, the cover image should be displayed on the page and its alt text is set to the value that's defined in the Airtable "cover_alt_text" field;
• If the workshop doesn't have a cover image uploaded in the Airtable, nothing is shown as the cover image on this page.

Expected behavior:

See above.