Add way to search on programming language

[david-a-wheeler]

The R community, in particular, want this on the form page /projects

[david-a-wheeler]

As also noted in #1209, we now have a much better reason to add this. One problem is that a lot of projects didn't provide this information. I just added implementation language information to 895 projects that didn't have it before. They're estimates from GitHub, which isn't always correct but is decent. That means that now 1582 / 1857 projects have information in the implementation_languages field (per "select count(*) from projects where implementation_languages LIKE '_%';").

[david-a-wheeler]

I was thinking that this could be done by splitting by comma on the field "implementation_languages" (after removing "(CII estimate)" if that's there), then adding each entry to the index with "implementation_language:" prefixed. E.g., if something uses R and JavaScript, you would have "implementation_language:R" and "implementation_language:JavaScript" in the index. I'm sure there are other ways to do it too, I just think it'd be good to have a way that would specifically use just the information from the implementation_language field.