Added skip_if_unavailable option to the config.repo file.

[ipanova]

closes #2902

[ipanova]

I am not sure I like this pattern adding config.repo options to both Repo and Publication models. Before I get to configurable gpgkey option, should we change what we're doing here? e.g. add a jsonfield to the publication/repo that will be storing the config?

If we do not foresee further requests to add more options to the config.repo we can probably leave this as is, especially if by default its generation will be turned off...but adding every time new field to 2 models is not sustainable.

[ggainey]

I think we'd be better off having a config.repo JSON field, where a user can set whatever they want the defaults to be. That would let us answer #2902, #2903, and #2985 all at once.

(Note from discussion Elsewhere - this is on Repo because at autopublish time we need it when we don't have a Publication, and it's on Publication because that's where it was originally)

[dralley]

replaced by #3298