Replace use of custom DJ matchers

* Sequence-dependent errors surfaced when using custom DJ shoulda matchers
* Replaced custom matcher with normal stub in ActivityCreatorJobSpec
* Replaced custom matcher with normal stub in InvitationCreatedMessageJob
* Replaced custom matcher with normal stub in EventCreatedEmailJob

Minor:
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* Prefer parentheses around method calls
Remove custom DelayedJob matcher
@wip Break long line
@wip Replace matcher in ReminderCreatedJob spec
Replace matcher for VoteCreatedJob
Replace matcher for VoteEmailJob
Remove DelayedJob::Matcher from spec_helper