Improve the audit trail for Miro image suppressions

[alexwlchan]

Currently information about suppressed Miro images is stored in two places:

• There's a manually populated spreadsheet in Google Sheets: https://docs.google.com/spreadsheets/d/1Ty_L8sivF1jILiRxt9u94xQKRUxwwBIGmMvYGDHVLv0/edit#gid=0
• When you run the suppress_miro.py script, it requires a message which is stored in DynamoDB

We should improve this process in the following ways:

• Provide an explanation of how Miro image suppression in GitBook, that explains when we'll remove images (i.e. when Collections Information ask)
• Merge the two lists of suppressed images, putting it all in DynamoDB
• Modify the suppression script to autogenerate a list of suppressed images that is human-readable, and link to it from GitBook

[alexwlchan]

I've copied all the old suppressions (and links to Slack etc.) into DynamoDB.