add an enhancement to show "become the FIRST x sponsor"

[mattstratton]

Currently, the code will display nothing for a sponsor type if there are no sponsors of that type. On many existing sites, if there is a "Gold" level, for example, with no gold sponsors, a link will be displayed that says "become the first Gold sponsor"

I think this can be handled by slightly changing the formatting of the sponsor data in the event data file, but it might require a little more cleverness in the parsing on the event template to determine which things to show.

[mattstratton]

Based on PR #38, this will now show "become an X sponsor" under every sponsor type, It also assumes a /sponsor link under the event page for this link to work. This is not perfect yet, but it's getting there.

[m1keil]

I think it's the way to go.
If you have unused sponsor_level, show it with a link to sponsors page.
We can do even an extra step and create some nice optional attributes under sponsor_level:
link_text - store the string that will be showed if no sponsors exists
show_link - boolean value to turn link on/off (if off, sponsor won't be showed)

[mattstratton]

I think that for now, this is fixed. We should add an enhancement to show "become the FIRST x sponsor" when the sponsor is empty, but I'm going to change this to an enhancement as it's not on the critical path.

[bridgetkromhout]

Related to #93.