Display current avatar in profile manager
Set umask to include execute flag for new directories
Use standard library for Filesystem::mkdirr()
Fix bug in starts_with()
Avatar paths derive from file hash

The scheme for avatar paths is now

<avatar_base_path>/<prefix>/<sha1_filehash_hex>.jpg

where <prefix> is currently N = 1 directory level with M = 2 characters
each of <sha1_filehash_hex>.
Print stacktrace on assertion failure in debug mode
Minor refactoring and automated whitespace correction