Since nearly the beginning, we've had manage.py check for required and

optional dependencies that could not be handled by the package's

dependency list, like git, cvs, etc. That code hasn't been updated much,

and had a lot of room for improvement. It also missed some of the more

recent mandatory dependencies.
This change fixes all this up, starting with adding dependency notices

for NodeJS, node_modules (and binaries within it), and removing some

legacy dependencies that are no longer worth checking for. The error

messages themselves have all been updated, giving some hints for

installation in order to help users deal with them.
The code for managing these dependency messages have moved from

settings.py to dependencies.py. This gives us a more formal (but

strictly internal) API for these types of checks, and lets us clean up

settings.py a bit. Along with the cleanup of those functions, one

check (for determining if settings.py lives within a reviewboard

directory) has been removed, as it hasn't really been relevant since the

mid-1.0 days.
These dependency messages have a much nicer display, prefixing any

required dependency errors with "ERROR:" and any optional dependency

warnings with "WARNING:". The actual text is wrapped to fit on the

screen, aligning after the ERROR/WARNING prefix, and adding a blank line

between each, helping to read the new content.

Tested each of the dependency issues (by forcing the display of each).

Verified the display and that URLs were correct. Also verified that if

any are shown, the final text with the manual URL was displayed.