[Self-hosting] Disable shared album discount code

[artemvmin]

When self-hosted, the discount banner at the bottom of any shared album is misleading for a couple reasons:

1. The banner implies that I’m a member of the hosted service, and that the person viewing my album should sign up to more easily share photos with me.

2. More importantly, I’m guessing these codes don’t work because they’re generated by my server. This is a poor user experience when they go to sign up.

I propose we either (a) add a flag to remove the banner or (b) update the wording to be appropriate for self-hosted instances.