Message Editing: Change default push rules to not send notifications for edits

[jryans]

We should change push rules so that notifications are not sent for edits by default.

I am not exactly sure how to best accomplish that though, as it's apparently hard to change the default push rules after account creation time. Anyway, some possibilities:

• Add a new default push rule to the spec that disables notifications for edits
• Allow Riot to install a user push rule that disables notifications for edits

Either way, we'd like to disable edit notifications by default. We could additionally allow them to be enabled for users that want that, but that doesn't seem required for a first pass.

[jryans]

@nadonomy brought up that we should think carefully about what happens to edits that introduce a mention of someone, etc. We may want to disable notifications for "basic" edits with no other features, but still keep them for edits that add a user mention.