Adds a new "HelpWantedCount" field to the Repository model.

This field is calculated by looking at all labels for all open issues,
and increments if strings "help wanted", "wants help", or "needs help"
are found. These labels can be any case and can contain punctuation;
i.e. "NEEDS_HELP" would also suffice.

The user interface can use this field to signal to the user that the
Repository has issues the organization would like help with.

Organizations can label their issues in this manner and consequently
more easily surface their needs for help to govcode users.

Addresses #29