Provide clearer explanation of what happened in notification email after an annotation is flagged

When a user flags an annotation in a group, the creator of the group is currently sent a [very terse email](https://github.com/hypothesis/h/blob/c480243e112e2cd5f54b7b65e40a6891ca57fbfe/h/templates/emails/flag_notification.txt.jinja2) which probably isn't going to make a whole lot of sense to someone who doesn't know about the moderation feature.

The text of the email should be revised to make it clearer what happened, why and what actions are available for the user to take.

At a minimum, it should probably make clear that:

- An annotation has been reported as inappropriate by a user in a group. It should probably specify which user, what group and perhaps include some details about the annotation.
- The user is receiving the email because they are a moderator of the group.
- The user can choose to hide the annotation.
- That the link shows the flagged annotation in context.

Read h/templates/emails/flag_notification.txt.jinja2, the linked current notification template, to understand the existing message. Revise the email so it explains the reported annotation, reporter, group, moderator role, available hide action, and that the link opens the annotation in context.

Written by the indexing model from the issue text.