* Add tests for relativeRoot

* Remove path.posix.join

Since this is for file system paths it feels incorrect to use it on
URL paths as they are different in many ways.

* Rewrite cookie path logic

Before we relied on the client to resolve the base given to it by the
backend against the path.

Instead have the client pass that information along so we can resolve it
on the backend.  This means the client has to do less work.

* Do not remove out directory before watch

This is re-used for incremental compilation.

Also remove del since that was the only use (and we can use fs.rmdir in
the future if we need something like this).

* Remove unused function resolveBase

The watch script was reloading the web server after every extension
compilation which is not necessary plus VS Code will not even be ready
at that point anyway.

Instead restart when the main compilation is finished.  The string to
match with includes a "with" because otherwise it would match "Finished
compilation extensions" which is not the main compilation task where we
actually need to restart the web server.

I also replaced this.log with console.log because the former does not
include a newline and it appears we want newlines with all
these (otherwise the next log starts on the same line which looks odd).

I removed the cache clean as well because the cache is meant to stay
there to speed up builds.

- Clean up watcher behaviors.

The "Starting watch-client" string no longer appears in the latest build
output.  We could look for "Finished compilation with" to avoid
restarting when other tasks restart (since they also include the name
i.e. "Finished compilation extensions with 0 errors") but I figure we
might as well restart code-server when any compilation task completes in
case other tasks include changes that need to be reloaded.

- Use yarn for vscode vendoring.
- Grab hash from package.

This way it can be used by the tests when spawning code-server on a
random port to look for the address.

This also refactors a couple CSS stylesheets to be referenced directly in the
HTML files.

And it removes any CSS imports from src/browser files.

Mostly to match everything else.

Mostly so the base URL resolution code can be shared.

- dashboard
- app api

Closes many issues that I'll prune after adding more docs
for users.