Task label feature

[ottointhesky]

First version of adding a task label feature that is queryable in the hubDB as discussed in #914.

What's missing?

• to check if task label is supported by all task submitting functions
• currently only dictDB supports the new column
• documentation of the feature (in docu and code)
• adding unittest

[minrk]

Nice! For applying the task_label metadata, look at how and where after is handled in the client, and copy that for a new label field (we can use label instead of task_label, since everything is a task already).