LEARNER-2172

Applied as a patch from pull #15773 by @nedbat

(cherry picked from commit 4201a315)

Refactoring: Use format with named variables

2. Add site configuration overrides to theming/helpers.py
3. Move microsite.get_value from theming/helpers to site_configuration/helpers
4. Move microsite_configuration.microsite.get_value usages to site_configuration.helpers.values

make password validation code more DRY

grammar nit: "less" -> "fewer"

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

ECOM-1956

The body of the enrollment message template

Tokenize platform name in message.

Changing to a datetime enrollment approach

Adding sorting. A little refactoring.

Adding confguration model for time delta

Adding admin registration and basic form for new config model.

Fixing docstring typo

Updating default time delta to 0, adding test to show it disabled functionality.

Removing the form from configuration and tweaking the enrollment message html

Added verification sidebar, banner for major courseware sections, quality & test improvements

Changed to subclassing django's PasswordResetForm and
overriding clean_password() instead of copy/paste.
Less lines to worry about for diff-cover this way =)

To following the link in the password reset email