allow users to leave a note when blocking a user

[abtinmo]

Hello there,

this PR references the following issue and Ames to implement a note feature for blocking the user.
#14397

I have implemented a test for POST method block-user, let me know if its needed to test the html response of manage-access and user/<name:user>/ as well.

one of the requirements was Show this in the UI when reviewing blocked users, and return it in the relevant API responses. i could not find any API that interacts with UserBlock model, please let me know if I'm missing something.

[nijel]

I don't think we have an API for this, so it looks complete for now. What I'd like to see is clarification in the help texts that this is an internal note not sent to the user. And please add a changelog entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.60%. Comparing base (8a12f18) to head (734ef5a).
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