Fix a grammar error

If two or more people like a submission, the website will show

"Foo and Bar thinks this looks great"

With this commit, the third person of the "think" verb is fixed:

"Foo and Bar think this looks great"
Merge pull request #2086 from whatyouhide/master

Fix a grammar error
Add redirect on nits index route
Add legacy redirect for old nitstats page
Don't attempt to delete a deleted submission
Don't include submissions by deleted users
Handle nil user in catch-all route
Handle non-existent exercise in like endpoint
Handle 404s from xapi metadata endpoints
Cannot edit non-existent comment
Add more context for difficult-to-reproduce exception
Don't try to reopen non-existent submission
Cannot edit comment on non-existent post
Warn about deleted submission if posting a comment fails
Mention lisp as active on site
Sort problems in language track order. Fixes #2098
Switch to Postgres for tests