This change modernizes some of the very oldest stuff left in our

codebase: repositoryform.es6.js. This has been replaced with a new model

and view, which will make it easier for scmtool/hosting service

subclasses to inject their own behavior and interaction.
The behavior here is the same, except for three bugs which I discovered:

The "repository plan" dropdown is now always repopulated, even when

  its row is hidden. Before this, with fake services or single-plan

  hosting types, the <select> was emptied, causing the form JS to look

  for a nonexistent #repo-form-hosting-<svc>-default element, and

  submitting the plan as null.
Fixed disabled-state computation for the submit button when switching

  to an SCMTool that is missing installed dependencies. This button had

  two different handlers managing its state (hosting-type != fake and

  tool == installed), which could conflict.
Reset "Edit credentials" back when the credentials sub-form was open

  and then the hosting service or account was changed.

Ran js-tests.
Edited some repositories and verified that behavior was unchanged

  (except for the fixed bugs).