Deprecate support for DSA keys

Overview

DSA keys have been deprecated due to weakness by OpenSSH, and we should wind down our support for these keys.

Approach

https://gitlab.com/gitlab-org/gitlab-ce/uploads/0ccee6f2ae874eb8879519001662b088/Screen_Shot_2018-05-26_at_12.44.19_PM.png

Disallow DSA SSH keys by setting the default setting in Visibility and access controls to disallow these keys (an admin can still override the setting).

OP by @rdavila

REF: gitlab-qa#183 (closed)

From @godfat:

@rdavila What do you think if we just drop support for DSA? It's been deprecated and considered unsafe anyway?

Me:

I think that makes sense given OpenSSH has deprecated it but this implies a change in our codebase and also we may need to communicate our users about it. cc: @DouweM

From @DouweM:

@rdavila Hmm, we should definitely have a separate issue to discuss that, so that we can properly time the deprecation etc. I wonder how many keys on GitLab.com that would affect. If you can come up with a query that'll find that information, I can run it on production :)

Edited by Jeremy Watson (ex-GitLab)