Fix broken docs search

[jpadilla]

Steps:

1. Click on search button at the top
2. Type search term
3. Hit return. "Searching..." text shows. No results show.
4. Hit return again. Results show up.

Problem seemed to be with how we were deferring requiring mkdocs search index after hitting return. I changed it to initialize after the modal's shown event has triggered.

[tomchristie]

Superstar! Thank you!