[#479] Update text to encourage user to log back in again w/codename

*Some* work for #479, based on a conversation I had this wk w/@dolanjs & @garrettr. Basically giving submitter as much guidance as possible on what they should expect, what we expect, what they should do next.

Wouldn’t mind some editing to make less wordy, but I think I hit all the places where some explanation/guidance could be used. Also as conversation in #479 notes, we could also describe where to keep the codename, etc. etc
Nit: Would be neater to put the shared notification message strings in global variables, since these are likely to change again in the future. Ex:
```
DOC_SUBMISSION_MESSAGE = "Thanks! We received your document "
CODENAME_REMINDER_MESSAGE  = "Please remember your codename: you can use it to log back into this site to read responses from us and to submit follow-up documents and messages."
MSG_SUBMISSION_MESSAGE = "Thanks! We received your message"
```
etc