Evaluate naming of `developer_key`, `developer_secret` fields

Right now, there are DB/model fields called `developer_key` and `developer_secret` on `ApplicationInstance`. They refer to _Canvas-only_ settings for integrating the Canvas Files Picker.

In Canvas-land, the exposed naming of this credential pair is "Developer ID" and "Developer Secret" (not "key"). So that's a bit funky, though confusion relief may come from the trivial #210 PR.

Wondering if these fields should be renamed or at least prefixed to make it clearer what they are. This may not be worth is because it would require a migration, etc., but wanted to take note of it in case it's a valid thing to deal with.