Up until now, we made the assumption that every hosting service just

needed a username and a password (which was sometimes an API key). This

isn't always sufficient, though, and we've worked around this limitation

in the past by taking per-repository fields for additional

credential-related information, which really should have been part of

the account details.
Now, hosting services can define an authentication form. This manages

fields for the hosting URL (if self-hosted), username/password, and

two-factor auth token. Subclasses can override the help text and labels

for these, override the fields completely, or create new fields.
The form is responsible for all validation for authentication

information, the transformation of the fields into a set of credentials

for authorization, and the authorization flow itself. It's also

responsible for state like which of the fields are required or presented

to the user (allowing us to remove some of the hacks in JavaScript).
The main repository configuration form now instantiates one of these

authentication forms for every hosting service. This means that for

every hosting service, the authentication fields are independent,

preventing errors from showing up in an auth form when moving between

hosting services, and preventing other such weird glitches we had with

sharing the auth form.
There's one UI change to how accounts for self-hosted services work.

Previously, the hosting URL field would appear before the accounts

field. You'd type the URL to the service, and the list of accounts

would update to show only those from that service. Now, we show all

accounts across all URLs, each showing the hosting URL its mapped to as

part of the name). The hosting URL is then shown below that (only when

linking an account). This helps make it clear what accounts you have.
The other major change is that re-authenticating now allows you to

change the username and hosting URL, if needed. Previously, this was not

allowed.
All-in-all, this change helps move us to a model where RepositoryForm

doesn't have to do all the work, and we're able to better split up

responsibilities. Similar changes will follow for repository and bug

tracker configuration in the future.

Unit tests pass.
Manually tested the following conditions:

Creating a new repository and account with no errors.
Creating a new repository and account with auth field errors, and then

  fixing them.
Creating a new repository and account with two-factor auth code required,

  and then entering it.
Creating a new repository and account with self-hosted service.
Creating a new repository with existing account.
Creating a new repository with existing account requiring re-auth, and

  providing new credentials (new username, new password, new hosting URL,

  new two-factor auth code).
Creating a new repository without a hosting service.
Creating a new repository with a hosting service using a custom auth

  form (repeating the above tests).
Creating a new repository with bug tracker configurations (both with

  new information and with using the selected hosting service's information).
Editing a repository (with all the above tests)