Suggest 'rake build' or 'rake db:migrate' when schema.rb is changed

[davidsbailey]

Today when we pull staging we get friendly reminders to run e.g. yarn or bundle install when necessary but not rake db:migrate. This PR adds a reminder to run rake db:migrate when schema.rb has changed.

Caveats

• someone might forget to commit changes to the schema, in which case we won't show a warning. this seems mostly harmless and is only as bad as it was before.
• we might someday have another file called schema.rb. but probably not.

Sample output

To test this I made a change to schema.rb from another git client, pushed it to origin, and then pulled it down to the original git client. Here is the output:

Davids-MacBook-Pro-2:~/src/cdo2 (suggest-db-migrate)$ git pull origin suggest-db-migrate 
From https://github.com/code-dot-org/code-dot-org
 * branch            suggest-db-migrate -> FETCH_HEAD
Updating 3806f63..693f564
Fast-forward
 dashboard/db/schema.rb | 16 ----------------
 1 file changed, 16 deletions(-)
dashboard/db/schema.rb changed; you probably want to run rake db:migrate or rake build

[aoby]

LGTM!

[aoby]

Do these "file patterns" accept paths? We could do "dashboard/db/schema.rb" just in case we ever get another schema.rb like you suggested.

[davidsbailey]

Currently these "file patterns" do not accept paths. They do an exact comparison to the result of File.basename. I don't want to risk breaking the rest of what's already working in this file so I think we should punt on that until we need it.