Prepare for db-driven auth configuration by making proviers operate in dual modes

Summary:
Ref T1536. This sets us for the "Config -> Database" migration. Basically:

  - If stuff is defined in the database, respect the database stuff (no installs have anything defined yet since they can't reach the interfaces/code).
  - Otherwise, respect the config stuff (all installs currently do this).

Test Plan: Saw database stuff respected when database stuff was defined; saw config stuff respected otherwise.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6240
Drive auth config with the database

Summary: Ref T1536. This is the last major migration. Moves us over to the DB and drops all the config stuff.

Test Plan:
  - Ran the migration.
  - Saw all my old config brought forward and respected, with accurate settings.
  - Ran LDAP import.
  - Grepped for all removed config options.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran, wez

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6243
Provide contextual help on auth provider configuration

Summary:
Ref T1536.

  - Move all the provider-specific help into contextual help in Auth.
  - This provides help much more contextually, and we can just tell the user the right values to use to configure things.
  - Rewrite account/registration help to reflect the newer state of the word.
  - Also clean up a few other loose ends.

Test Plan: {F46937}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6247
Add Amazon and Asana providers

Summary: Ref T1536. Ref T2852. Adds providers for Asana and Amazon. See D6248.

Test Plan:
{F46960}

{F46961}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536, T2852

Differential Revision: https://secure.phabricator.com/D6249
Minor, fix the "password" auth default (should be enabled if not provided).
Minor, fix an issue where creating a provider without changing anything
fails to save it because there are no effective transactions.
Minor, clean up an obsolete hardcode (no password registration).
add support for adding and removing reviewers in createcomment conduit method