Add and improve types in src/sidebar/config/ so that this directory
typechecks with `noImplicitAny` enabled.

 - Add "$rpc:requestGroups" as a possible value for the `groups` config
   in types/config.js, which code in sidebar/config/ needs to replace
   with a promise for a list of groups.

   This required a change in `sidebar/services/groups.js` to either
   check for or explicitly assert that `service.groups` is an array at
   this point (and not the string "$rpc:requestGroups").

 - Replace various `object` types with `ConfigFromHost`,
   `ConfigFromSidebar` or `SidebarSettings` as appropriate

 - Rewrite the code at the bottom of `hostPageConfig` that copies
   configuration from the `config=` URL fragment into a sanitized
   `ConfigFromHost` object, so that it is easier to read and add types
   to.