Modify login page to remove OAuth options and add disclaimer about public data

[servilla]

During an EDI team meeting, it was decided that the login page for the Data Portal should:

1. add a statement and link to ezeml about being the place to go to prepare data for submission to EDI
2. not have any of the OAuth provider options for logging: since an OAuth authentication only provides access to data requiring an "authenticated" user and there are so very few of these data - as is, the OAuth options lead to user confusion
3. explicitly state that 99% of all data products do not require a login to access
4. add a link from the hover-over alerts about possibly needing to be logged in (from the landing page) to a separate (and mostly hidden) OAuth login page for only the possible cases of requiring an "authenticated" user

see here: https://docs.google.com/drawings/d/1comvfcNOeY8JyQiCpK78QPzb2wFsodZB1hU-vSRRgpY/edit