Add support for changing the actions for default rules

[NegativeMjark]

See matrix-org/matrix-doc#283

Works by adding dummy rules to the push rules table with a negative priority class and then using those rules to clobber the default rule actions when adding the default rules in list_with_base_rules

[dbkr]

Yep, as you say IRL, may want to strip out any remaining rules with prio class -1 or otherwise handle this, although of course it's perfectly safe until we remove a default rule! Probably should do the check now though rather than leaving a heffalump trap for someone. Otherwise lgtm.

[NegativeMjark]

I've removed all the rules with negative prio class, b3fb790

[dbkr]

lgtm, merge it! :)