Fix google image search engine

- Because there is not full image url in the dom, we replace "image_url" with the same url as the "url" (url of source).
  See example HTML https://gist.github.com/Nachtalb/2dea8a4d2c723c49226ad9645838121f
- Remove unused import
- Fix google image search title
- Keep google image safe value up to date

[fix] DuckDuckGo Images requests

In order to use another cache tool or to use spot without cache
system.

The new redis version is more strict about the kind of object
to store, we cannot use the same way that with the version 2.X.
In redis we want to store the SearchData class. Instead of
store field one by one we store the full serialized object with pickle.

Pickle is not very secure but we dont care for now because the redis
storage will be deploy in the same host that spot service. In a second time
we can use another way to serialize SearchData with msgpack and mashumaro
for example.

Use HTTPS for crossref

[fix] get YouTube results

[fix] Stop engine from sending duplicated requests

SearchData is composed by SearchQuery field, so we use inheritance
in order to use the same field naming. In that way we can use the an
uniq way to build our redis key.

Locale and search language was always defined with english value.

This patch inits the locale on `pre_request` in order to define the
default value of locale and language preferences.

Plus the `best_match` function provided by flask babel library did not
work as expected. So the function `match_language` provided
by searx is used to detect that the language from Accepted-Language
header can be used in searx project.

args parameters are merged in form in pre_request, so
this patch removes that useless operation.

With SEARX_LOGGER setting we can run spot with
the expected logger level.

Results are not appearing in the html document anymore,
instead they are found inside an object embedded in a script.

Update footer with my-spot url and version

See merge request !30