Move DKIM keys to database

[Deltachaos]

This PR solves issue #2949.

Moving the DKIM keys to database removes the requirement for shared storage in the admin.

Currently a migration path from files to database is missing. My idea is it to add this to the migration file, but i am not to deeply into the python framework here to know if this is a good idea or not. If you have some tips i happily add this feature so that this PR can get merged.

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[nextgens]

Ah; I have code for doing this on a branch somewhere...

IMHO we can't merge it as is: we want to address the other related DKIM tickets too: #2687 #2618 #2053 #2050 #1700 maybe even #1519

We need a separate table (as the same keys could be used cross-domains -like for ARC-), we probably want additional fields (keytype, usage, generation timestamp, status, ...)

[Deltachaos]

I see. But wouldn’t having the keys i the database even without changing the current behaviour that much, make a migration to the changes you mentioned easier?

[nextgens]

I see. But wouldn’t having the keys i the database even without changing the current behaviour that much, make a migration to the changes you mentioned easier?

I don't mind incremental improvements... but the database schema should be as close to the end-state as possible. The last thing we want is to have to lost information in the process and make migration more complex as a result.

At the very least, we want a separate table with a column for the selector, a timestamp it was generated/imported on, and a way to link it to the domain it was enabled for (keeping in mind that there may be multiple DKIM keys)

[nextgens]

Here is what I had on my branch if you need inspiration:

0001-V1.txt

class Key(Base):
    """ A Key used for DKIM or ARC """
    __tablename__ = 'key'

    name = db.Column(db.String, primary_key=True, nullable=False)
    key_type = db.Column(db.Enum(KEY_TYPE), nullable=False)
    key_size = db.Column(db.Integer, nullable=False)
    private = db.Column(db.String, unique=True, nullable=False)
    public = db.Column(db.String, unique=True, nullable=False)
    generated = db.Column(db.DateTime, nullable=True, default=datetime.utcnow())
    active = db.Column(db.DateTime, nullable=True)
    retired = db.Column(db.DateTime, nullable=True)
    use_for_dkim = db.Column(db.Boolean, nullable=True, default=False)
    use_for_arc = db.Column(db.Boolean, nullable=True, default=False)
    """ a misnamer: Domain or Alias that makes use of it """
    domains = db.relationship('Domain', secondary=key_mapping,
            lazy='dynamic', back_populates="keys")