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. Testing Done: - Ran js-tests. - Edited some repositories and verified that behavior was unchanged (except for the fixed bugs).