Add “change password” shortcut on the JupyterHub main page

### Proposed change
Add a "change password" button to the JupyterHub navigation bar, which points to the already existing ".../hub/auth/change-password" page. (Similar to what nativeauthenticator has.)

### Alternative options
I don't really see an alternative. Most web pages that require a login, also provide an easily accessible "change password" button and/or field, so it would be good to stick with that expectation? ;-)

### Who would use this feature?
Every user of a JupyterHub instance (e.g. TLJH) that utilizes the firstuseauthenticator. Currently, it's not obvious, that a change password functionality even exists. (And users IMHO do not likely stumble across it themselves via e.g. reading the community docs.)

### (Optional): Suggest a solution
Implementing this might be as simple as using [this template](https://github.com/jupyterhub/nativeauthenticator/blob/9d03a277319ca0f44cee2bfb879d659feb7786d9/nativeauthenticator/templates/page.html) e.g. from the nativeauthenticator?