Convention for names of tables and columns

[2chanhaeng]

While #101, I added the DB schema. However, while the existing schema names followed camelCase, the newly added schema names followed snake_case. It was my mistake, but @dahlia suggested that since snake_case is conventional for table and column names, we should leave the ones added in #101 as they are and rename the existing ones. They left this suggestion as a comment on #101.