* Prevent race from causing remote clients from being closed
`web.SessionContext` was susceptible to races retrieving `auth.ClientI`
for remote sites. The mutex on the remote client cache was not held for
the duration of the `GetUserClient` which meant two simultaneous requests
could find no client existed in the cache, create one and then add it to
the cache. When the second client is added to the cache it would close the
first client created, which results in the `use of closed network connection`
error displaying on the web UI as described in #17173.
To remedy this the portion of `GetUserClient` that retrieves the remote client
is now protected by a `singleflight.Group`. This ensures that races on creating
a client no longer happen.
`web.SessionContext` was slightly refactored to make it easier to test, to
remove some unnecessary operations, and to provide `context.Context` to `GetUserClient`.
Additionally, all `web.SessionContext` variables which were named `ctx` in `lib/web`
have been renamed to avoid confusion as that often refers to a `context.Context`.
Fixes#17173
rosstimothy