Allow to tag directories

**Motivation**
Like files and resources, we want to be able to tag directories

**Acceptance criteria**
1. The user must have the write permission
2. The tag must be valid
3. The tagged revision must exist

Start by locating the existing tagging implementation for files and resources, then trace how directories, permissions, tags, and revisions are represented. The work is done when directories can be tagged only with write permission, a valid tag, and an existing tagged revision, with the relevant tests passing.

Written by the indexing model from the issue text.