Merge 'custommails' into develop: "readme!"

This update contains state_migrations on `scipost.remark` and
 `scipost.{vgm_tables}`. This means the database scheme won't be
 altered but the migrations table which tracks all migrations done
 by the system will be updated to make sure the system knows how
 to handle the new apps. (Note the db_table option in the models of
 the new apps, this makes sure no db scheme migration is needed).

When doing the migration, the following will be asked:
"""
The following content types are stale and need to be deleted:

    scipost | newsitem
    scipost | motion
    scipost | nomination
    scipost | feedback
    scipost | vgm

Any objects related to these content types by a foreign key will also
be deleted. Are you sure you want to delete these content types?
If you're unsure, answer 'no'.
"""

One can anwser 'yes', the database won't be altered, no entries will
 be lost during this operation.