Standard implementation of signing task validator

[HauklandJ]

TODO: what should this entail? Input from TEs needed.
Thoughts: Minimum required signatures - count/everyone? Validate hash - if mismatch delete signatures? In practice, all signatures associated with a sign task are deleted on task entry, so this should not be necessary as data from previous tasks should be locked.

[HauklandJ]

@ivarne Is validation strictly on task end, or does validation exists on patches as well?

[ivarne]

Incremental validation (those that have relevant changes) happens on PATCH, so that frontend can “patch” its issue list.