Change filename length restriction to 60 characters & word truncation

I have hard time explaining migrations in 20 characters or less. Longer filenames are also more searchable. This changes filename length restriction to 60 characters and truncates the filename by words and not in the middle.

Before: 4443a5b8a43_alter_user_table_tel.py

Example of a longer filename: 3e12cdb9209d_alter_user_table_telephone_and_mobile_columns.py

Example of a filename that has been truncated: 12894284029g_create_partial_index_for_article_telephone_column_and_.py