Add a search bar

I don't actually know how to write PHP. So I just guessed/improvised.

My thought was that there would be an option in the admin section for showing search (which sets a show_search variable? Is that how you do that?).

I also don't actually know what this widget looks like, so I can't say if this will stylistically fit.

The search bar is from the [Bike Index HTML plugin](https://github.com/bikeindex/stolen_bikes_widget_html). Right now it only does serial search, but when the search API is updated it will do even more cool things (and you won't need to update the code to get the new features).

**Also I closed the `h5` tag**