Clean up redundant methods in datetime field formatters

Problem/Motivation

Follow-up from #2161337: Add a Date Range field type with support for end date. There are now quite a few datetime field formatters, and many of them have redundant methods that are either exact duplicates, or duplicates that have grown out of sync as not all have been updated equally (see, for instance the ::viewElements() methods).

Proposed resolution

• All datetime field formatters to extend from a single base class
• Remove redundant methods in child classes

Remaining tasks

User interface changes

API changes

Data model changes