hide sponsor form link if it's undefined, fixes #10

[phette23]

also edit a few if conditions to use != nil, ref #8

testing:

• open up /prospectus/ and the final paragraph under Impact of Support should show the sponsorship email but not the form link
• extra credit: comment out the sponsorship email & the email there as well as the yellow "contact" sidebar should both disappear

[dzoladz]

Tested changes made by @phette23. Those modifications function as described.

While review those updates, I noticed a context button that was visibly hidden because it lacked the context button stylings. I've added a few directives to make the button appear on the interface and folded in the logic to hook it to the existence of `{{ site.data.conf.sponsor-btn-link }}'

[phette23]

Good catch @dzoladz! I can confirm your changes work (button doesn't display without btn-link value, if you add it the button displays with appropriate styles). The online conference sentence is also gone.

Looks like you can't review your own PRs but I sign off. I won't merge in case we want a third party to review first.

[dzoladz]

As this one is a low impact change, and we've already reviewed each other's work, I don't think it's critical to have a third-party review.

I'll merge this one into main before creating a PR to open proposals.