cli: implement new share-remove command

[tiborsimko]

For the workflow sharing sprint, design and implement an initial unshare command allowing to unshare workflow runs with colleagues with whom they previously shared it.

This issue is basically a mirror of #680.

For example, Alice unshares her myanalysis run 42 with Bob:

$ reana-client unshare -w myanalysis.42 --user bob@example.org
myanalysis.42 is now unshared from bob@example.org

Beware of giving nice error messages when asking to unshare with a colleague with whom this analysis run was not shared, and a proper exit status:

$ reana-client unshare -w myanalysis.42 --user bob@example.org
==> ERROR: myanalysis.42 was not shared with bob@example.org previously. 
$ echo $?
1 

Beware of giving nice error messages when users don't exist:

$ reana-client unshare -w myanalysis.42 --user bob@example.org
==> ERROR: The user "bob@example.org" does not exist. 
$ echo $?
2

Note different exit status codes for different kind of error situations.

P.S. This issue depends on reanahub/reana-db#206 and provides a mirror functionality to the share command implemented in #680.