Enable users to delete their accounts

Related issue: https://github.com/hypothesis/product-backlog/issues/355

**Problem you are trying to address with this feature**

In addition to changing username, we receive a number of requests from users to delete their accounts. The ability to delete an account, should go hand-in-hand with the ability to create an account, allowing users to feel in control of their data, which includes the ability to remove it.

Upon deletion it is suggested that we have a way to gather valuable feedback from users by asking them either by a short survey e.g.

We're sorry to see you go, would you mind telling us why you are deleting your account today?

[] I don't use Hypothesis
[] I'm concerned about my privacy
[] Other (please specify)

Any other ways we could improve Hypothesis?

--

_Questions I have:_

Deleting an account, removes all annotations and replies associated with this account. Is this correct? i.e. there will be no orphaned annotations? (Other services substitute the original username for 'a user' or 'former member' etc. thus preserving the integrity of a previous user's data.

Start by reviewing related issue 355 and the five-comment discussion, then resolve the open questions about whether annotations and replies are deleted or anonymized. Done should include an agreed account-deletion flow and a decision on whether to collect the proposed exit survey feedback.

Written by the indexing model from the issue text.