Before now table definitions could define collations only for MariaDB using the
MariaDB's collation names directly.
Now instead definitions get a slightly more abstract collation name syntax, but
only supporting the collations utf8mb4_bin and utf8mb4_unicode_(cs|ci) (wrapped
as utf8_bin, utf8_general_(cs|ci)), because those are the ones that have
practical use for GNU social.
Which also means that on MariaDB the formerly used utf8mb4_general_(cs|ci) have
been superseded by utf8mb4_unicode_(cs|ci), as they are the more modern
replacement.
Introduce collation support on PostgreSQL which results in use of the C (POSIX)
collation as utf8_bin and the und-x-icu collation as utf8_general_cs.
utf8_general_ci is also mapped to und-x-icu, which makes it case-sensitive,
unfortunately.
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