Show repeat groups in column selection dropdown

Above the submissions table, we show a dropdown that allows the user to select the columns to display in the table. We do not show repeat groups within the dropdown. However, this might be confusing, especially if there are fewer than 12 other fields: in that case, there won't be an indication that the table does not show all the data. Instead, we would like to:

- Show repeat groups (fields whose `type` property is `'repeat'`) in the dropdown.
- But fields inside repeat groups should still not be shown.
- And the checkbox for the repeat group should be disabled. The `title` attribute should read, "In this version of ODK Central, you cannot view repeat groups in the browser."
- The total number shown in the select element ("# of #") should include the repeat groups shown in the dropdown.
- The dropdown should always be shown if the form has a repeat group, even if there are fewer than 12 other fields.