Archive access point

[somexpert]

This purpose of this PR is to add an access point to archived URLs from within the app. fixes #1

[ldko]

In this PR we could include a change to url_listing.html where if the project's archive_url has been given a value, we display under the "URL" (url entity) an "Archived URL" heading and a link to what should allow access to the url in the archive.

[somexpert]

@ldko, @vphill, @jason-ellis, @jthomale, this is ready for review

[ldko]

We should probably support both base urls entered with the slash on the end or not.

[somexpert]

will do

[ldko]

I think we could do something with the logic of:

    def clean(self):
        if self.archive_url and not self.archive_url.endswith('/'):
            self.archive_url = '{}{}'.format(self.archive_url, '/')

Also, I think we need blank=True for the archive_url field assignment in the model either way.

[somexpert]

Ok, I'll add that in there and test it out. And do we need the blank=True since we have the null=True?

[ldko]

I believe we need the blank=True in order to make it not a required input in forms (including the admin).

[somexpert]

alright, I'll add that in there too

[ldko]

:+1:

[jason-ellis]

:+1: