Unified style and accessibility upgrades for login, registration, and profile pages

[joemull]

Closes #4380.

Implementation notes

• Brought all pages where a user manages their own account into the back office, and deprecated the theme templates
• Adjusted the admin base as needed to handle unauthenticated users
• Implemented a contextual title element so that the relevant site name is appropriately provided in addition to the page title
• Wrote view tests to make sure new templates are hit
• Used "secondary" style buttons for accessibility (color contrast)
• Adjusted link colors for accessibility (color contrast)
• Improved form accessibility in a few places
• Made heading structure consistent, though I would have preferred not to use h2 for these elements (h1 is taken by the header and used for the site title, which is not ideal in my opinion)

Limitations

• We have got to sort out the heading structure in core/base.html -- it results in two h1s in most cases, and one of them is often empty. My preference is that h1 is used for the actual page, not the site name, and that the site name is simply appended to the title element, so it is available in the browser tab.
• The interests field is a consistent accessibility trap, until we can get away from jquery / tagit. We may be able to re-implement this bit of the page as a Django formset