Admin badge interface improvements

Validations are added when saving a badge, to eliminate several common causes of badge queries failing. And a dashboard notice is added to alert the admins when they do fail.

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Commit message:

> Upon saving a badge or requesting a badge result preview,
BadgeGranter.contract_checks! will examine the provided badge SQL for
some contractual obligations - namely, the returned columns and use of
trigger parameters.

> Saving the badge is wrapped in a transaction to make this easier, by
raising ActiveRecord::Rollback on a detected violation.

> On the client, a modal view is added for the badge query sample run
results, named admin-badge-preview.
The preview action is moved up to the route.
The save action, on failure, triggers a 'saveError' action (also in the
route).

> The preview action gains a new parameter, 'explain', which will give the
output of an EXPLAIN query for the badge sql, which can be used by forum
admins to estimate the cost of their badge queries.
The preview link is replaced by two links, one which omits (false) and
includes (true) the EXPLAIN query.

> The Badge.save() method is amended to propogate errors.

> Badge::Trigger gets some utility methods for use in the
BadgeGranter.contract_checks! method.

> Additionally, extra checks outside of BadgeGranter.contract_checks! are
added in the preview() method, to cover cases of null granted_at
columns.

> An uninitialized variable path is removed in the backfill() method.

> TODO - it would be nice to be able to get the actual names of all
columns the provided query returns, so we could give more errors