Tracking issue: queueboard filtering hacks

[grunweg]

This issue tracks a few mild hacks for the dashboard filtering. When writing a custom search function (cf. #7), these should be removed in facour of a smarter custom search features.

• the author column is searchable via author:name --- this is a string appended to the author column, hidden from view
• the author column is searchable via assignee:name --- for each assignee (if any), there is such a string appended to the assignee column, hidden from view
• the labels column gains an invisible text label:t-algebra$ just for t-algebra PRs