[ci] Add git-cop configuration file

git-cop configuration file has to be copied in:
`~/.config/git-cop/configuration.yml`

The enabled cops with error severity are:

- `commit_author_email`: Ensures author email address exists.

- `commit_body_leading_line`: Ensures there is blank line between the
  commit subject and body.

- `commit_subject_prefix`: Ensure that the commit has one or more tags
  (*api*, *app*, *backend*, *ci*, *dist*, *doc*, *webui*)
  alphabetically ordered.

The enabled cops with warning severity (a message is render when
executing git-cop but it doesn't fail) are:

- `commit_body_bullet`: enforce `-` for lists.

- `commit_body_line_length`:  Ensures each line of the commit body is
  not longer than 72 characters.

- `commit_body_phrase`: Avoid meaningless words/phrases.

- `commit_body_presence`: Ensures the commit has a description.

- `commit_subject_length`: Ensures the commit subject length is not more
  than 50 characters.