Centralize string literals for e.g. account form variables

Right now we use the string names for each form variable in account creation/update in at least 3 places. Here is an example for "first_name", which is used in the actual form set-up, such as
``
as well as in the javascript validation, like
`rules { first_name: "required",`
AND in the python processing of the form, as in
`ACCT_FNAME_KEY: body['first_name'],` .

This is fragile so we should centralize. Doing so means we need to define each field name constant somewhere in python, then pass each constant into the templates that use it and put it into the html--and relevant javascript!--with `{{ }}` syntax so that it is guaranteed the same everywhere it is used. That kind of change makes the templates harder to read and maintain, so we'd want do to it thoughtfully.

Start by locating the account creation/update form setup, JavaScript validation, and Python form processing described in the issue, then inventory every duplicated field name. Done means the shared field definitions are used consistently across the HTML, JavaScript, and Python paths without changing account-form behavior.

Written by the indexing model from the issue text.