FilterHelper: allow a column to be enwrapped in a function
Add a MySQL function which retrieves if a semver version is stable
allow filtering item list by stable/unstable semver version
items/list: add filters for minimum and maximum semver version
yippie! drastically improve performance when sorting/filtering semver

Using MySQL's MEMORY table engine is much faster.
add version patterns for stdlib release listing
fix error messages (copy-paste)
implement "stable" filter for item description

When specifying special versions ("latest" or "first"), one can
now specify to use only stable or unstable versions with an new
additional filter.
rework publish filter for release descriptions

Set the default to BOTH, but let it be set in a parameter.
Require authentication not based on the parameter, but on
the actual release.
Merge branch 'semver-filters' into master