Use PhabricatorCustomField infrastructure to render Releeph custom fields on the edit screen

Summary:
Ref T3718. This moves custom field rendering on the edit screen to PhabricatorCustomField and makes all the APIs conformant.

We still run through edit with both old-school and new-school sets of fields, because the actual editing isn't on the new stuff yet. That will happen in a diff or two.

Test Plan: Edited a request; intentionally introduced errors and verified the form behaved as expected.

Reviewers: btrahan, testuser1122344

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3718

Differential Revision: https://secure.phabricator.com/D6756
Add `src/extensions/` to Phabricator

Summary: I added this easier-extension mechanism a while ago but only added the actual directories to libphutil and arcanist. This one should work, it just wasn't checked into the repository.

Test Plan: Added a file with an exception in it to the directory, verified the exception was thrown at runtime.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6759
Use ApplicationSearch in ReleephBranchView

Summary:
Ref T3721. Releeph currently attempts to implement a flexible, field-driven search for branches, but it's building all of its own infrastructure and it ends up heading down some weird paths. In particular, it loads **every** request and then makes calls into fields to filter them. It also tries to be very very general, which isn't really necessary (for example, I think it's reasonable for us to assume that we won't let you disable the "requestor" field).

ApplicationSearch and CustomField provide more scalable approaches to this problem; move search on top of them. The query still ends up doing some filtering in-process, but it's now far more limited in scope and can be denormalized later.

Test Plan: {F54304}

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T3721

Differential Revision: https://secure.phabricator.com/D6758
Conpherence - fix conpherence create

Summary: somewhere along the line this broke. Before this patch we fail the visibility check since its based on Conpherence Participants which don't get created and attached until applyExternalEffects. Believe it or not, this was the least gross fix I could come up with; since the permission check is done SO early most other ideas I had involved creating a dummy participant object to pass the check then handling things for real later on...  Ref T3723.

Test Plan: created a conpherence with myself - great success

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, Korvin, aran

Maniphest Tasks: T3723

Differential Revision: https://secure.phabricator.com/D6762
Log and continue when trying to register an invalid event listener

Summary: We currently die if an event listener throws when registering (e.g., because it is misconfigured), but this prevents you from running `bin/config` to fix it, which is a bit silly.

Test Plan: Created this revision with an invalid listener in config.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6761
Add src/extensions/ to .gitignore

Summary: Oops -- forgot to do this in D6759.

Test Plan: `git status`

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6760