management: introduce replacerelations command

management: introduce replacerelations command

The replacerelations script is used to ingest relations into Patchwork's
patch database. A patch groups file is taken as input, which on each
line contains a space separated list of patchwork ids denoting a
relation. All the existing relations in Patchwork's database are removed
and the relations read from the patch groups file are ingested.

Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
[dja: pep8, drop relations directory as empty dirs don't get stored by git,
      comment about how lines are generated.]
Signed-off-by: Daniel Axtens <dja@axtens.net>