Fix bug creating poem with 0 or 1 poets

When a poem is created with 0 poets, the poets form field is undefined.
When a poem is created with 1 poet, the poets form field is an integer.
Convert the value to an array with the handy [].concat function.
Exclude poets from typeahead that have already been added to the poem.

Use the typeahead filter function to exclude items that are already in the list. This approach has the limitation of potentially returning fewer than 10 results. In a worst-case scenario, the top 10 poets starting with a given letter could be added to the poem, and 0 results would be returned! Another approach could be to pass the list of poets to the server and exclude them with a SQL query. This would probably be prohibatively expensive from a performance standpoint.
Add the user to the new poem by default.

Render the user in the poet list on the new poem page. Remove the server-side logic that adds the current user to the poem, as this is effectively done on the client now. There is now some conceptual duplication between the server and the client, because the jade template and the jquery logic both need to know the structure of the li and the containing input.

Also changed the wording of the placeholder to "Add other poets"