Check for expired and revoked keys

[dethos]

This PR handles the following issues:

• Fixes a bug that lets the users submit a key, without fingerprint check
• Add revoke and expiration checks to settings form
• Adds a daily task to update and check the users keys (Issue: Update keyserver keys #1)

[nunosilva800]

I'm not very good with Python but LGTM.
I think at least the fingerprint check should be covered by tests.

[dethos]

@Onumis Agree, the whole project needs to have a good coverage of tests. Probably we should give some priority to this #15 issue, before advancing to the next features.

[dpflucas]

Seems OK to me. Will merge this, agreeing that from now on we should focus on having features shipped with tests.