Summary:
Fixes T13031. "Enormous" changes are basically changes which are too large to hold in memory, although the actual definition we use today is "more than 1GB of change text or `git diff` runs for more than 15 minutes".

If an install configures a Herald content rule like "when content matches /XYZ/, do something" and then a user pushes a 30 GB source file, we can't put it into memory to `preg_match()` it. Currently, the way to handle this case is to write a separate Herald rule that rejects enormous changes. However, this isn't obvious and means the default behavior is unsafe.

Make the default behavior safe by rejecting these changes with a message, similar to how we reject "dangerous" changes (which permanently delete or overwrite history) by default.

Also, change a couple of UI strings from "Enormous" to "Very Large" to reduce ambiguity. See <https://discourse.phabricator-community.org/t/herald-enormous-check/822>.

Test Plan: Changed the definition of "enormous" from 1GB to 1 byte. Pushed a change; got rejected. Allowed enormous changes, pushed, got rejected by a Herald rule. Disabled the Herald rule, pushed, got a clean push. Prevented enormous changes again. Grepped for "enormous" elsewhere in the UI.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: joshuaspence

Maniphest Tasks: T13031

Differential Revision: https://secure.phabricator.com/D18850