[fix] make /stats more CSP compliant - bar graph

Replace bar graph's *styles* (see below) by CSP compliant implementation in
``searx/static/themes/__common__/less/stats.less`` ::

    ./simple/stats.html:49: <span style="width: calc(max(2px, 100%*{{ (engine_stat.result_count / engine_stats.max_result_count )|round(3) }}))" class="stacked-bar-chart-serie1"></span>{{- "" -}}
    ./simple/stats.html:57: <span style="width: calc(max(2px, 100%*{{ (engine_stat.http / engine_stats.max_time )|round(3) }}))" class="stacked-bar-chart-serie1"></span>{{- "" -}}
    ./simple/stats.html:58: <span style="width: calc(100%*{{ engine_stat.processing / engine_stats.max_time |round(3) }})" class="stacked-bar-chart-serie2"></span>{{- "" -}}
    ./oscar/stats.html:50:  <span style="width: calc(max(2px, 100%*{{ (engine_stat.result_count / engine_stats.max_result_count )|round(3) }}))" class="stacked-bar-chart-serie1"></span>{{- "" -}}
    ./oscar/stats.html:58:  <span style="width: calc(max(2px, 100%*{{ (engine_stat.http / engine_stats.max_time )|round(3) }}))" class="stacked-bar-chart-serie1"></span>{{- "" -}}
    ./oscar/stats.html:59:  <span style="width: calc(100%*{{ engine_stat.processing / engine_stats.max_time |round(3) }})" class="stacked-bar-chart-serie2"></span>{{- "" -}}

Reported-by: https://github.com/searxng/searxng/issues/57


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