Make automatic suggestions immutable

[fricklerhandwerk]

Currently our implementation allows actually removing derivations from automatic suggestions. This destroys data which we will need in the future to improve suggestion quality.

As @RaitoBezarius originally proposed, instead let triaging workflows merely overlay a filter on derivations and maintainers. The activity log will also need to be adapted to track changes on the filter and display them accordingly. Then an issue draft will need to be a separate data model which can be written to, and then converted to a proper issue record when publishing.

In short, the metadata we add manually to an automatic suggestion is:

• accepted or rejected?
• derivations and channels affected (this says something about names and versions)
• maintainers affected (this is not relevant for matching though)