CPC batch editor cleanups

[StephenChan]

Follow-up on PR #453.

• Link to the batch editor page somewhere, perhaps from the CPC annotations upload and CPC export forms.
• Clean up CSS, particularly spacing between elements.
• Write more tests in Python.
• Write tests in Javascript, using QUnit like our Browse actions tests.
• The Help page on CPCe operations could use updating. Link the batch editor from here.
• Update the browser compat info in our docs to 1) clarify that FileSystemDirectoryReader.readEntries() DOES seem to be supported in Edge, despite MDN not saying so; 2) stop listing compat info for IE; and 3) update for more recently-used features. Actually, this probably entails discarding the existing tables entirely and just leaving a single row for readEntries().