Create basic 404 error page
Insert header into 404 error page
Add dependency for JSP servlets
Add i18n to 404 error page
Add bootstrap3 CSS/font files + publish them + include in 404.html
Put in static files under _static/, and simplify url_rewrite rules

The reason to put under _static/ is that we possibly have to add a
rule in front-end webserver, better to chose a name that will remain
and contain all possible static files (as bootstrap ones).
Update doc
Improve page with minimal explication text (feel free to extend)
use "body" key, not "text", as in ldapadmin
Fix encoding + remove unused strings
Manage 403 error
detail
indentation
Merge remote-tracking branch 'origin/13.09' into errorpages
Move error templates to config, indicating they could be customized
Merge pull request #525 from georchestra/errorpages

sec-proxy: add 404 and 403 error pages