feat: rework file handling

### REST

* changed file download path from `running_exercise/<team_id>/<file_id>/` to `files/<exercise_id>/<file_id>`
* changed file upload path from `running_exercise/upload/<team_id>/` to `files/<exercise_id>/`

### Additions

* added field `attachments: [FileInfoType!]!` to `ContentType` type
* added field `uploadedBy: RestrictedUser` to `ContentType` type
* added field `uploadedAt: DateTime!` to `ContentType` type
* added field `exerciseId: Int` to `ContentType` type
* added field `fileIds: [UUID!]` to `CustomInjectType` type
* added field `fileIds: [UUID!]` to `CustomInjectInput` input
* added field `fileIds: [UUID!]` to `SendEmailType` type
* added field `fileIds: [UUID!]` to `SendEmailInput` input

### Changes

* changed field `definition` type from `DefinitionType` to `ExerciseDefinitionType` on `ContentType` type

### Deletions

* removed field `fileInfo: FileInfoType` from `ContentType`
* removed field `fileId: UUID` from `CustomInjectInput`
* removed field `fileId: UUID` from `CustomInjectType`
* removed field `fileId: UUID` from `SendEmailInput`
* removed field `fileId: UUID` from `SendEmailType`

Closes #389