Implement task/batch bidding process

[cfedermann]

Users should be able to select tasks to work on.

The list of available tasks should be filtered as follows:

• a maximum of three users is allowed to work on a single task;
• after selecting a task, the user either has to finish or retract from it;
• the task list should also display language pairs;
• only language pairs that are "valid" for the current user should be shown.

[cfedermann]

This is basically done; will need some more testing after the data is ready...

[cfedermann]

Works out nicely, hence closing this issue.