Add report field to allow linking reports to other reports

[preaction]

We should have a way to link different reports together for various reasons. The most common reason will be "same underlying issue", but another reason could be "caused by upstream issue" or "causes downstream issue".

We need to add a new links array to the report JSON format. Each entry in the array should be a link object with the following keys:

• type - This is the type of link. It should be "report", but future values will be possible. Different types of links will need different validations.
• report_id - The ID of the linked report
• reason - The reason for linking the report. Should be one of:
  • related - A generic "related" to be used for reports showing the same issue. "duplicate" is not entirely accurate
  • causes - The linked report shows the cause of this report. When the issue in the linked report is fixed, the issue in this report will be fixed.
  • caused_by - The linked report was caused by the issue in this report. When the issue in this report is fixed, the issue in the linked report will be fixed.
• comment - A markdown field for a general comment about the linking

The causes/caused_by may not be that useful: There are systems in place to prevent reports from downstream modules from being submitted if an upstream module fails its tests, but those may not always work.

These links will likely not be filled in by reporters, but will be filled-in later by users on the website.