Add cache feature on valid http requests

Closes #50

See merge request !58

Redis LRU cache is added on http requests, only
response with a valid status code are cached during 1 day.

Close: #50

Close: #1830
Signed-off-by: Nicolas Gelot <nicolas.gelot@e.email>

Previously, to display images and videos we had performed
a lot of changes in Searx project which leads to hard rebase
to upstream, then it adds a latency on each request to load
the main page.

Now, images and videos requests are asynchronous, these ones are
called from JS code with the Searx REST API.

Close: #45

TLS configuration is done in a config file. Waiting for
https://github.com/containous/traefik/issues/5507 to avoid
to mount a volume.

Report fix from https://github.com/searx/searx-docker/commit/b4ce81ab604df1fe5bc6e5d8666e542335c665d0

On dev mode we dont want use tor as proxy by default. With
env. variables these settings are only available on prod.

Close #48

[mod] infinite scroll: XHR requests are sent with the header "Accept: text/html"

Full CSP support and filter bot with filtron service

Closes #44

See merge request !57

Close: #44

Close: #44

Ref: #44

Close: #33

Ref: #33

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.

Close: https://github.com/asciimoo/searx/issues/844
Close: #35, #42

searx.be stopped using Fathom analytics