Feature: Configurable open signups

[avinassh]

This PR lets you have complete open sign ups and is configurable with an env var. Set OPEN_SIGNUPS=true, any other string or absence of this env will be considered as false. So, existing Lobsters instance will run as expected.

Changes:

1. If OPEN_SIGNUPS is not set or set to any value other than true, app works like earlier.
2. If OPEN_SIGNUPS is set to true, in the header there will be an item Sign Up Now and in footer Join <site name>, they both point to /invitations/open.
3. I added a new CSS class signup-text which can be used to customize Sign Up Now text in header.

[avinassh]

Screenshot:

[avinassh]

@pushcx any update on this PR?

[pushcx]

Thanks!