The `H_SERVICE_URL` environment variable is only used for one thing: to
set the value of the `defaultSidebarAppUrl` variable (to
https://hypothes.is/app.html in production or localhost:5000/app.html in dev)
by appending `/app.html` to the end of the environment variable's value.

For consistency with the config setting and variables that it's used to
control, rename `H_SERVICE_URL` to `SIDEBAR_APP_URL` and require the
user to include the `/app.html` ending in the environment variable's
value.

Also rewrite the documentation for this environment variable to make it
clearer what it's used for.

**Note**: This change means that developers who're using the
`H_SERVICE_URL` environment variable in their development environments
will need to:

1. Rename `H_SERVICE_URL` it to `SIDEBAR_APP_URL` in their environment
2. Append `/app.html` to the end of the environment variable's value

For example:

    export SIDEBAR_APP_URL="http://localhost:5000/app.html"

The developer docs for this environment variable have been updated to reflect
this.