Implement what happens when comment period is closed

[jehlers]

• [x] Hide sidebar links to "write a comment"
• [x] Changing the preamble introduction (https://github.com/eregs/notice-and-comment/issues/106#issuecomment-212603007) and
• [x] Update the homepage (https://github.com/eregs/notice-and-comment/issues/183#issuecomment-220681553) to mention the closed comment period
• [x] Taking away the date from the right sidebar and replacing it with "Comment period has ended" (we punted #191)
• [x] Allow the write tab to be clicked on, but replace write mode with a message that explains the comment period is over and sends users to the docket folder with all the comments (https://github.com/eregs/notice-and-comment/issues/236#issuecomment-217993811)

[jehlers]

Design for the last checkbox from #236:

Allow the write tab to be clicked on, but replace write mode with a message that explains the comment period is over and sends users to the docket folder with all the comments

[jehlers]

For the third bullet above, here's a design for what we can do when the comment period closes on the homepage:

• Leave countdown at 0
• Add the red ! icon next to the end date
• Change "Ends" to "Ended"
• Disable the button / swap out for closed message and link to just reading the proposal

cc @cmc333333

[cmc333333]

I think @jmcarp's implemented some of this. We'll need to validate

[cmc333333]

@tadhg-ohiggins is handling the home page part

[cmc333333]

All merged in