Add SQL migration that drops the old FTS4 MessageSearchTable table,
re-create as a FTS5 table, clean up the column names a bit, and adds a
flags column so unread/starred queries can be made fast.
Define a SQLite FTS5 extension function `geary_matches()` to replace
the FTS3 `offsets()` function which no longer exists in FTS5, based on
Tracker's implementation.
Update code to FTS5 conventions (docid -> rowid, etc), use new column
names, populate and update the flags column as the email's flags
change, and use new match function for getting matching tokens.
Advanced searches are probably currently broken, these will be fixed
by subsequent commits.
