* fix context's SiteURLHeader to use the subpath
This fixes various commands such as /open and /groupmsg, as well as
OAuth related redirects.
* MM-11246: also leverage GetSiteHeader() in completeSaml
* factor out GetSubpathFromConfig
* mv web/subpath.go to utils/subpath.go
* serve up web, api and ws on /subpath if configured
* pass config to utils.RenderWeb(App)?Error
This allows the methods to extract the configured subpath and redirect
to the appropriate `/subpath/error` handler.
* ensure GetSubpathFromConfig returns trailing slashes deterministically
* fix error 404 handling
* redirect /subpath to /subpath/
This is necessary for the static handler to match, otherwise none of the
registered routes find anything. This also makes it no longer necessary
to add trailing slashes in the root router.
* Refactor context out of API packages
* Update function names per feedback
* Move webhook handlers to web and fix web tests
* Move more webhook tests out of api package
* Fix static handler
