This ensures that events are created when wiki pages are
created/updated/destroyed in the app.

This does not add events for wiki pages edited off-line and then
published using git-access.

The wiki page base service is the place where WikiPageMeta objects are
constructed, leaving the logic in `EventCreateService#wiki_event` very
thin. For this reason the two related changes are contained in a
transaction.

We also refactor wiki services to avoid passing statically known
parameters. This is a more OOO conformant design, with fewer branches
and if statements. The different kinds of actions are distinguished,
with three actions:
 - external_action (used in the webhook)
 - usage_counter_action (used in the counter)
 - event_action (stored in events)

These have (potentially) different values and different types.

This interface in WikiPages::BaseService is enforced by implementing
the three methods inheriting classes must override.

We also place feature flag checks around more use sites. This ensures
that we do not create wiki page events if the flag is turned off, and
that we do not filter for them either.

Includes suggested changes from reviewer (@.luke)

Co-authored-by: @.luke

Incidental chang: Less dynamic queries

This passes class names for polymorphic types as constant strings. We do
this because these strings are persisted to the database, and changes to
the application need to keep previously stored values in mind. Any
updates to the WikiPage::Meta class name need to ensure that values
stored with `"WikiPage::Meta"` as target_type will still be around.