Extension manager: Implement extension table

This uses a lot of code and the whole design from the previous extension
manager implementation.

style improvements

enable email notifications & alert messages for imported users

removed a merge conflict I missed on merge

added basic suport for embedding (html5) videos

de/de-informal/en: Removed redundant suffixes in TOC of configuration
manager / Synchronize terms in de and de-informal

check less compilation for errors

This now gives proper files and line numbers for errors

improve html structure and styling for add & edit user notes

made necessary by PR#254 which adds content below these notes.

usermanager: fix an issue where edit user table would incorrectly show the password blank message

add csv import functionality to the user manager

de/de-informal updates: Replace "Nutzer" with more common word "Benutzer"
/ fix several typos / Fixes FS#2821

Enable LANG.template.<template>.key to provide localized strings in templates

Extension manager: implement uninstall, canModify and install/update

Extension manager: Improve update check

Extension manager: Use getInfo() when no info.txt is available

Extension manager: fix install dir for templates

small fix for pagetools in RTL languages

Revert "translation update"

This reverts commit d6d855093f24c6c3f608d005189385959a7efbf7.

I guess it was a bit too early to think straight :-/

translation update

prevent overlong select boxes (FS#2817)

Brazilian Portuguese language update

added default icon to pagetools

de/de-informal: localization updates (delete user function)

Extension manager: add language file and simple admin component

Extension manager: implemented more extension info and basic repository access