[fix] simple-theme templates: polish HTML <img> elements

- <img> [1] element does not have an end tag, no need for a leading `/>`
- add global attribute title[2] to result_templates/images.html (result-images-source)
- in jinja macro 'macro result_header' remove duplicate of class="image"
- in jinja macro 'macro result_header' remove alt attribute (fix [3])::

    alt="{{ result.title|striptags }}"

  the result.title is already shown in::

    <h3>{{ result_link(result.url, result.title|safe) }}</h3>

[1] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img
[2] https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes#attr-title
[3] https://github.com/searxng/searxng/issues/523

Closes: https://github.com/searxng/searxng/issues/523


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>