Sort and Filter attributes

[arnese]

There should be more attributes to filter within the search form both for public and own requests (https://github.com/okfde/froide/issues/133). They could include:

• [ ] Costs (https://github.com/okfde/froide/issues/152)
• [ ] People interested, either by number or on/off (https://github.com/okfde/froide/issues/151)
• [ ] Public / non-public
• [ ] Tags
• [ ] Sorting: Date of last message, Date of first message
• [ ] Project / no project (https://github.com/okfde/froide/issues/249)
• [ ] Limiting: Show 20/50/100 results?
• [ ] Filter by rejection reason
• [ ] My requests / other people's
• [ ] Request includes documents / request includes filetype / request includes machine-readable data
• [ ] Make geosearch possible