Pick minor fixes from searxng (#3251)

* [fix] Rename ccengine engine to openverse

The CC engine was merged with WordPress and renamed to Openverse

Source: https://wordpress.org/news/2021/05/welcome-to-openverse/

* [fix] ccengine engine - avoid unwanted redirects

api.openverse.engineering is a little picky and wants to have a trailing slash
in the path:

    /v1/images? -->/ v1/images/?

otherwise it redirects, here is the debug log:

    DEBUG   searx.network.openverse       : HTTP Request: GET https://api.openverse.engineering/v1/images?&page=1&page_size=20&format=json&q=foo "HTTP/2 301 Moved Permanently" (text/html; charset=utf-8)
    DEBUG   searx.network.openverse       : HTTP Request: GET https://api.openverse.engineering/v1/images/?&page=1&page_size=20&format=json&q=foo

 "HTTP/2 200 OK" (application/json)
    WARNING searx.engines.openverse       : ErrorContext('searx/search/processors/online.py', 105, 'count_error(', None, '1 redirects, maximum: 0', ('200', 'OK', 'api.openverse.engineering')) True

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

* [fix] FutureWarning from lxml

Just in case if content is None, the original code will skip extract_text(), and
just append the None value to 'content'. So just add allow_none=True, and this
will return None without raising a ValueError in extract_text().

* [enh] Add pagination to Brave

Also added ```&spellcheck=1``` because now it is disabled by default, not returning any ```suggestion_xpath```.

Co-authored-by: Léon Tiekötter <leon@tiekoetter.com>
Co-authored-by: Markus Heiser <markus.heiser@darmarit.de>
Co-authored-by: capric98 <42015599+capric98@users.noreply.github.com>
Co-authored-by: Allen <64094914+allendema@users.noreply.github.com>