Schema update

[hlageek]

Facilitate requal updates:

• On project load, perform a check of requal version schema to the existing project schema.
• If some tables are missing, create them.
• SQL queries and create_db_schema optimized to support the above features

Closes https://github.com/RE-QDA/docs/issues/41.

Possible future improvements:

Create missing columns.

Requal version columns can be extracted by regex, e.g.:

tbl_cols_matches <- stringr::str_remove_all(db_call[1], "[\n\r\\s]") %>%
stringr::str_match_all("[\\(\\,]([a-z_]+)")
tbl_cols <- unique(tbl_cols_matches[[1]][,2])