[fix] make /preferences more CSP compliant - simple & oscar theme

in simple & oscar theme replace bar-graph's *styles* by CSP compliant
implementation in::

    searx/static/themes/__common__/less/stats.less

Fixed files::

    ./oscar/preferences.html:29:        <span style="width: calc(max(2px, 100%*{{ (stats[engine_name].time / max_rate95)|round(3) }}))" class="stacked-bar-chart-median"></span>{{- "" -}}
    ./oscar/preferences.html:30:        <span style="width: calc(100%*{{ ((stats[engine_name].rate80 - stats[engine_name].time) / max_rate95)|round(3) }})" class="stacked-bar-chart-rate80"></span>{{- "" -}}
    ./oscar/preferences.html:31:        <span style="width: calc(100%*{{ ((stats[engine_name].rate95 - stats[engine_name].rate80) / max_rate95)|round(3) }})" class="stacked-bar-chart-rate95"></span>{{- "" -}}
    ./simple/preferences.html:43:        <span style="width: calc(max(2px, 100%*{{ (stats[engine_name].time / max_rate95)|round(3) }}))" class="stacked-bar-chart-median"></span>{{- "" -}}
    ./simple/preferences.html:44:        <span style="width: calc(100%*{{ ((stats[engine_name].rate80 - stats[engine_name].time) / max_rate95)|round(3) }})" class="stacked-bar-chart-rate80"></span>{{- "" -}}
    ./simple/preferences.html:45:        <span style="width: calc(100%*{{ ((stats[engine_name].rate95 - stats[engine_name].rate80) / max_rate95)|round(3) }})" class="stacked-bar-chart-rate95"></span>{{- "" -}}

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


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