Contributions: add filter capability on custom fields

[micsucmed]

The "contributions" page doesn't allow to filter by custom fields, as it is possible with abstracts. This would be extremely useful to have in contributions as well (e.g., filter out all contributions having the abstract already QA'd, or special cases/groups of abstracts).

Please note that under abstracts we have a blue "Customise list" button, while under contributions we have a grey "Filters" one. The two act for the same function, while in contributions the blue button is "New contribution". This is quite user-unfriendly.

Note: custom fields are even not exported so it's not even possible to "filter" contributions externally (e.g., Excel)

Note: check list generation in registration filters, should be similar