Support configuration-driven custom fields

Summary:
Ref T1702. Ref T3718. There are a couple of things going on here:

**PhabricatorCustomFieldList**: I added `PhabricatorCustomFieldList`, which is just a convenience class for dealing with lists of fields. Often, current field code does something like this inline in a Controller:

  foreach ($fields as $field) {
    // do some junk
  }

Often, that junk has some slightly subtle implications. Move all of it to `$list->doSomeJunk()` methods (like `appendFieldsToForm()`, `loadFieldsFromStorage()`) to reduce code duplication and prevent errors. This additionally moves an existing list-convenience method there, out of `PhabricatorPropertyListView`.

**PhabricatorUserConfiguredCustomFieldStorage**: Adds `PhabricatorUserConfiguredCustomFieldStorage` for storing custom field data (like "ICQ Handle", "Phone Number", "Desk", "Favorite Flower", etc).

**Configuration-Driven Custom Fields**: Previously, I was thinking about doing these with interfaces, but as I thought about it more I started to dislike that approach. Instead, I built proxies into `PhabricatorCustomField`. Basically, this means that fields (like a custom, configuration-driven "Favorite Flower" field) can just use some other Field to actually provide their implementation (like a "standard" field which knows how to render text areas). The previous approach would have involed subclasssing the "standard" field and implementing an interface, but that would mean that every application would have at least two "base" fields and generally just seemed bleh as I worked through it.

The cost of this approach is that we need a bunch of `proxy` junk in the base class, but that's a one-time cost and I think it simplifies all the implementations and makes them a lot less magical (e.g., all of the custom fields now extend the right base field classes).

**Fixed Some Bugs**: Some of this code hadn't really been run yet and had minor bugs.

Test Plan:
{F54240}
{F54241}
{F54242}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1702, T1703, T3718

Differential Revision: https://secure.phabricator.com/D6749
Partially move Releeph custom fields to PhabricatorCustomField

Summary:
Fixes T3661. Ref T3718. This makes Releeph custom fields extend PhabricatorCustomField so we can start moving over other pieces of infrastructure (rendering, storage, etc) to run through the same pathways. It's roughly the minimum amount of work required to be able to move forward.

NOTE: This removes per-project custom field selectors. Fields are now configured for an entire install. My understanding is that Facebook does not use this feature, and modern field infrastructure has moved away from selectors.

Test Plan: Viewed and edited projects, branches, and requests in Releeph. Grepped for removed config. Grepped for `field_selector`.

Reviewers: btrahan

Reviewed By: btrahan

CC: LegNeato, aran

Maniphest Tasks: T3661, T3718

Differential Revision: https://secure.phabricator.com/D6750
Simplify and improve PhabricatorCustomField APIs

Summary:
Ref T1703. Ref T3718. The `PhabricatorCustomFieldList` seems like a pretty good idea. Move more code into it to make it harder to get wrong.

Also the sequencing on old/new values for these transactions was a bit off; fix that up.

Test Plan: Edited standard and custom profile fields.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1703, T3718

Differential Revision: https://secure.phabricator.com/D6751
Further simplify PhabricatorCustomFieldInterface

Summary:
Ref T1703. Ref T3718. This introduces `PhabricatorCustomFieldAttachment`, which is just a fancy `array()`. The goal here is to simplify `PhabricatorCustomFieldInterface` as much as possible.

In particular, it can now use common infrastructure (`assertAttached()`) and is more difficult to get wrong.

Test Plan: Edited custom fields on profile.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1703, T3718

Differential Revision: https://secure.phabricator.com/D6752
Make ReleephRequest implement PhabricatorCustomFieldInterface

Summary: Ref T3718. Doesn't do anything yet.

Test Plan: Viewed and edited a request.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3718

Differential Revision: https://secure.phabricator.com/D6753
Partially fix a policy issue with ApplicationTransactions

Summary: Currently, we check that the user can view and edit their own transaction, which is always true. Instead, check that they can view the object. I'll fix this with a more tailored check against the EDIT capability that's per-transaction later.

Test Plan: Applying no transactions no longer fatals with undefined `$xaction`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6754
Delete "Risk" field specification from Releeph

Summary: Ref T3718. This is not used and does not seem particularly useful.

Test Plan: Grep.

Reviewers: btrahan

Reviewed By: btrahan

CC: LegNeato, aran

Maniphest Tasks: T3718

Differential Revision: https://secure.phabricator.com/D6755