Build the default database path from STORAGE_DIR

Currently, the default database path is hardcoded to /opt.  In a
development installation, graphite-web may be installed anywhere
and will fail to start following the development installation
instructions.  Using STORAGE_DIR aligns this value with the
comments earlier in the file.

- Changed definition of DATABASES

- Moved DATABASES definition to after STORAGE_DIR definition.
Don't overwrite local_settings.py defined DATABASES

As per the discussion in PR #937.
URL shortener was added to 0.9.13 release notes
Merge pull request #939 from deniszh/master_release_notes

URL shortener was added to 0.9.13 release notes
Fixing release notes after #941
Merge pull request #943 from deniszh/master_release_notes

Fixing release notes after #941
Merge pull request #937 from andrewbaxter/settings_default_db_path

Build the default database path from STORAGE_DIR