Catch ALL exceptions and enwrap them

By default, a release is based on the latest created release
(the version number is based on it). Now the base can be spe-
cified to be all releases (default) or the latest published
release.

New class to handle stdlib releases in one central place.

Use new StdlibRelease class to simplify code for release
description and creation.

Do not declare update types statically - use UpdateType::getCode().

Pass the DB connection to the constructor. Split up into further
methods. For filters of type 'switch', allow a set of conditions
to be met. Validate comparison parameters. Add a new 'expr' type
and some structuring comments.

Use the modified FilterHelper class to re-implement the last pa-
rameter of the Suspension class. Adjust the filter code for can-
didate listing.

Implement the last candidate filter with FilterHelper.
Adjust all calls as necessary.

Was added in ae78166.

A coerce callback must be specified.

WARNING: This changes the outdated behaviour that the 'user'
filter took user names instead of IDs.

The old MySQL extension is deprecated and causes warnings in newer
PHP versions. Thus, switch to the new recommended mysqli extension.

Switch from MySQL to mysqli

This reverts commit 1d690a9 as it breaks
the Suspension::isSuspended() method.

Pull recent bug fixes to make tests run fine.

Once again pull fixes to make the tests pass.