Load buddy images from URL.

This allows the frontend proxy to cache images and avoids hitting the backend server too often. This also is required in preparation for further changes to move user data away from memory.
Get configuration from "mediaStream" service.
Remove user images on disconnect.
Load buddy images from "static" subfolder.
Append filename to image url.
First path component when loading buddy images contains flags (currently unused).
Set width/height for buddy image (currently hardcoded to 46 pixels).
Set alt tag of buddy image to user name.
Only change update index if image actually changed.
Include timestamp of last change in URL instead of human readable update index.
Use 15 random bytes as image id so we don't get base64 padding.
Update and evaluate caching headers when returning buddy images.
Hardcode buddy image resolution in template.
Also store absolute url as "buddyPictureLocalUrl" to be used in chat views.
Added method "mediaStream.url.buddy" to get url of buddy image.
Leave "alt" tag empty.
Merge pull request #15 from fancycode/buddy_image_urls

Load buddy images separately