Add zh-CN and fix some problems

1. Add zh-CN for the project.
2. Add language support for tinymce.
3. Fix some string not i18n friendly and add some strings to en-GB.php,
so the other language file should add these strings.
4. Other bugs

Known problems:
1. When deleting an experiment or an item, the page says “Deleting This
?” this string is in \js\common.js and \js\common.min.js, don’t know how
to make it support i18n.
2. When already installed eLabFTW and delete config.php, running
install.php will get an error: SQLSTATE[23000].
3. Many pages use google fonts and other online resources, so when
offline there will be some problem.
4. Default template and datepicker is English.
5. When exporting pdf and zip file, Chinese characters are substitute to
”_”。