Feature: LDC-254: contest rules agreement

## Ready State: Yes

This pull request is related to the following JIRA tickets:
https://imagex.atlassian.net/browse/LDC-254

Displays contest rules field data on contest entry node create form & enforces a checkbox to be checked before form can be submitted. This is enforced for all users.

## Requirements & Dependencies

None

## Deployment Instructions

* Merge code.
* New imagex_contest & imagex_contest_permissions features must be enabled
* Clear caches with `drush cache-clear all`

## Usage/Test Instructions

* Create a contest node
* Ensure you see a "Submit Your Entry Now" link to the bottom of the contest node, clicking the link allows you can create a contest entry node with the "Related Contest" field prepopulated
* The bottom of the screen ensure the "Rules" inputted in the contest node are displayed along with a checkbox before the form action buttons. 
* Ensure you cannot submit the form until the checkbox is checked

#### Notes

* The rules only show on initial node creation form not edit
* For none admin users the "Related Contest" field is hidden, if it has no value users are redirect to previous page
* I tweaked the "Submit your entry now" link as part of this work (maybe not the best place) so it shows different messages to the user depending on their access & whether the contest is still open to entries