Make elements in the sidebar collapse able.  Except infoboxes all elements in
the sidebar are collapsed by default.

By folding out the sidebar elements, the UI looks less cluttered.  Especially on
small devices like smartphones, where the sidebar is above the results list, the
UX should be improved [1].

[1] https://github.com/searxng/searxng/issues/2140

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

- <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>

This makes it easier to separately handle search and index requests
from a web server or from a reverse proxy.

If a request to index contains a query, a permanent redirect HTTP response
is returned. This should give some level of backwards compatibility
for users that have set a searx instance in their browser's search bar.

The new url parameter "timeout_limit" set timeout limit defined in second.
Example "timeout_limit=1.5" means the timeout limit is 1.5 seconds.

In addition, the query can start with <[number] to set the timeout limit.

For number between 0 and 99, the unit is the second :
Example: "<30 searx" means the timeout limit is 3 seconds

For number above 100, the unit is the millisecond:
Example: "<850 searx" means the timeout is 850 milliseconds.

In addition, there is a new optional setting: outgoing.max_request_timeout.
If not set, the user timeout can't go above searx configuration (as before: the max timeout of selected engine for a query).

If the value is set, the user can set a timeout between 0 and max_request_timeout using
<[number] or timeout_limit query parameter.

Related to #1077
Updated version of PR #1413 from @isj-privacore