Clean up the "wordform" model's properties

[eddieantonio]

The Wordform model represents a wordform in the source language.

A wordform should have the following fields or properties:

• [ ] is_head: this wordform has explicit defintions. is_head == True iff the wordform has one or more definitions
• [ ] word_class or specific_word_class — which could be a foreign key that references a SpecificWordClass model

A wordform should no longer have the following fields:

• [ ] as_is
• [ ] pos

TBD for the following field:

• [ ] is_lemma: a lemma is the representative wordform for a lexeme. The codebase as of 2021-05-19 does not use the is_lemma field to mean this; however, each wordform's lemma field is a foreign key that references the representative Wordform; lemma representative wordforms reference themselves (self reference). Django magic allows you to do my_wordform.inflections to get all stored inflections of the lexeme!

@dwhieb please confirm that my linguistic terminology is accurate!

@andrewdotn, does this make sense? We can pair on it if you'd like!