Simple URLs for communities

## Feature Request Form
### Problem you are trying to address with this feature

Currently, our group URLs look like this:
`https://hypothes.is/groups/Rj7eVijj/our-group`

Because groups have been private, we've wanted to make it easy for people to create group names irrespective of whether others have exactly the same name, i.e. groups are not namespaced. So I can make a group called "Dans English 101" and someone else can choose the same name and there's no issue. That's a good thing.

Note: The last part (`our-group`) is actually not a required part of the URL, and is discarded by our software (you can change the string and still get to the same group). It's included simply as a convenient visual way to distinguish one URL from another at a glance.

However, there are two problems with this for more public community groups:

1. Communities will not want to be just casually distinct from one another, but strictly distinct from one another. For instance, there should only be one Climate Feedback community--partly to ensure that another community does not spin up squatting the same name but with a different approach to annotating the same material, but also simply so that when you say "Climate Feedback" people know what you mean. While an easily distinguishable URL may not mean that another community couldn't have their community home page *claim* to be Climate Feedback, and won't prevent that other community from choosing a URL which *appears* to be close, a distinctive URL is a solid first step.

2. There's a value to a simple, clean URL that serves as a good home for a community, and that users can easily remember (and type in by hand if need be-- for instance, any subreddit URL, as long as I can remember the name of the subreddit, I can usually type in directly w/o issue).

### Your solution

For example, a clean URL might look like: `https://hypothes.is/groups/OurGroup`
Or possibly: `https://hypothes.is/h/OurGroup` (each authority we host could have its own simple, namespaced URLs under it's own path).

These simple urls for groups should be case insensitive (which might preclude the first option).

Creation of this kind of a unique, namespaced group should probably be something that communities can request, perhaps providing a rationale for why it's in the public interest, and that h (or authority) staff have to approve first.

No implementation files, tests, or entry points are identified. First clarify the URL namespace, case-insensitive uniqueness, and community approval workflow with maintainers; done means an agreed design and implementation scope for unique, memorable community URLs.

Written by the indexing model from the issue text.