[#51] Add permission to administer Redirect settings

[juampynr]

For #51.

Based on #65.

[juampynr]

@Berdir is there anything missing in this pull request that I should work on?

[Berdir]

Usually, an administer permission can do everything. That's a pretty common pattern. I'm not sure I like changing that.

What if we turn it around and introduce something like a "manage redirects" permission and update the titles/descriptions so that it's clear that administer allows access to the settings and redirects, while the second permission only allows to manage redirects?

[juampynr]

That makes sense to me. Updating pull request now.