models: Validate Project.linkname does not contain forward slash

models: Validate Project.linkname does not contain forward slash

I started by creating a project that contained a forward slash
(importing patches from https://lists.sr.ht/~sircmpwn/sr.ht-dev/) and
it fails to render the "projects" main page.

The specific error reads:

    NoReverseMatch at /

    Reverse for 'patch-list' with keyword arguments
    '{'project_id': 'foo/bar'}' not found. 1 pattern(s) tried:
    ['project/(?P<project_id>[^/]+)/list/$']

which appears to explicitly disallow forward slashes.

So I think it makes sense to validate that project linkname doesn't
contain forward slahes.

This implementation uses the validate_unicode_slug validator instead of just
rejecting inputs that contain forward slashes.

Signed-off-by: Thomas Bracht Laumann Jespersen <t@laumann.xyz>
Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #380