Readying poster module for rollout.

* Removing mention of the Symposium and student logins from home page.
* Adding a techfest login to the nav bar.
* Removing student signup button from the welcome page.
* Fixing poster new redirect variables after failed new poster validation.
* Changing poster submit button to "Upload poster to job".
* Validating poster variables so name/title is required, making length
* between 20 and 40 inches, and making width 20 inches or more.
* Changing hints on poster new page to explain validation requirements.
* Formatting the file_field input for the new poster page, making it
* look bootstrappy.
* Adding error message to file field if errors exist.
* Creating helpers to check for and show file field errors.
* Creating a delete job button in the new poster page.
* Making the back button on the new poster page to redirect to the job
* show page.
* Fixing the verbage on the mounting hint on the poster form.
* Making attachment urls for poster show pages open in a new tab.
* Changing mounting field in poster show page to Yes or No.
* Removing attachment urls for faculty/students/etc.
* Changing "Save Poster" to "Confirm Poster" for Poster show view.
* Making poster show views open in a new tab from job show page.
* Adding an extra finalize button to the bottom of the job show page.
* Adding an extra confirm poster button to the bottom of the poster show
* page.
Making attachment urls open in a new tab from poster index page.