Review Board's gaining worker support, and already differs when run

backed by a web server or via a script, but until now we've had no way

of telling what mode Review Board is in.
This change adds a process_mode= argument to initialize() for

specifying a mode. This default to default, which is used for scripts

or other operations, but can be put into server or worker mode.
Right now, there's no difference between the modes, but going forward

we'll be able to alter setup or key off some functionality based on the

mode. For example, servers will be able to launch workers, but scripts

and workers will not.
The current mode is available via

reviewboard.process.get_rb_process_mode() and via the

$RB_PROCESS_MODE environment variable.
There's a bit of cleanup in the initialization while here to avoid

redundant or unnecessary calls. Some of this may be updated based on the

process mode in a later change.

All unit tests pass.
Ran locally and saw that Review Board was initialized in server mode.