Added some Git integration

- New menuitem in admin menu for arbitrary Git commands
- Handler in action executes 'git ' . `escapeshellcmd(...)`
- Notify of command and result
- Requires admin user

Due to the limitiations of PHP + Git, many commands are unavailable.
- Without setting up a user + SSH key, only git:// & http(s):// works
for push, pull, etc
- Possible permissions issues relating to Apache vs logged in user
- As command is made available through prompt() in JavaScript, no
tab completion is available.
- There is currently no way to gather more input from user after
command is executed. Would require WebSockets to do so, if even possible
- Commands such as log, diff, etc may be truncated or not be shown at
all
due to length restrictions.