Adding metadata

## Is your feature request related to a problem? Please describe.
Adding metadata to files/folders and maybe even spaces which can be searched by the user to locate the file or folder.

## Describe the solution you'd like

1. As an owner of a space/file/folder I want to add multiple tags (key-value) to a space/file/folder. By default the owner of a space/file/folder can create tags for the space/file/folder (s)he owns.
2. As an owner of a space/file/folder I want to be able to delegate reshare, read, write, edit, delete permissions to other users.
3. By default the owner has full control over the tags referring to the space/file/folder (s)he owns.
4. Tags can be searched and the search returns the space/file/folder the tags refer to.
5. A tag must be unique system-wide.
```diff
+Just to avoid 1324 tags called "myproject"
```
6. The owner of a space/file/folder can share that space/file/folder with others and also with certain permissions. The same holds for tags. The permissions on tags **may** or **may not** be the same as the permissions on spaces/files/folders.
```diff
+Suggestion: Make them the readonly by default just like with spaces/files/folders and allow the person who shares the space/file/folder and tags to adjust the permissions on tags later on.
```
7. All tags that are **not** readable by an user **must not** be searchable by that user.

## Describe alternatives you've considered
They are not there

## Additional context
This is a starting point for a discussion among the OCIS early adopters and developers. It is not like everything needs to be just as described above. But I would like to have peoples opinion and ideas. For starters, I would stay away from schema-based metadata like dublin core etc. That would be a whole new subject. Just add metadata that makes it easy to retrieve data associated with a certain topic.