Allow blocklists to block files.

**Is your feature request related to a problem? Please describe.**
People shy away from some zites because they suspect/fear illegal content. While it is possible for blocklists to block user(-id)s and entire zites, it is not possible to block specific files on zites which can be an issue depending on the architecture of the zite in question. An example for that would be millchan(the non merger version) and avatar images on ZeroMe(because they will be marked as required and thus be downloaded onto everyone computer in the case of CP this can be devastating).

**Describe the solution you'd like**
Introduce the ability to block files from specific websites as an ZeroFrame api command. One way of doing that would be to have ziteAddress, date_added, inner_path, name and reason in a filters/files.json. If the user then tries to download a muted file, a notification will appear, which informs him of the reason why that file is blocked, asks if he wants to get it anyways and an option to not show that notification again. Since the user should always be able to tell which files are blocked on his end, blocking files based only on its hash should not be the implemented.

**Describe alternatives you've considered**
Blocking users and/or zites in the case of millchan (and the theoretical case of ZeroMe). Both don't work satisfactory or at all.

**Additional context**
This issue is mirrored from the [GitCenter ZeroNet repository](http://127.0.0.1:43110/1GitLiXB6t5r8vuU2zC6a8GYj9ME6HMQ4t/repo/issues/view/?1P4w4Rvh4vS9r6G5xTiSh8qEW87EKZtnJB/5@1DfrA2M9Qra6stqT6tzwNMYNweswY54KAC)(you need to install ZeroNet to visit this link)

Start with the ZeroFrame API command path and the proposed filters/files.json structure named in the issue. Trace how file downloads and notifications are handled; done means specific files can be blocked per zite, the reason is shown, and the user has the requested override controls.

索引モデルが issue の本文から書いたものです。