Primary action button consistency (Tracker)

Created by: rrhyne

Our primary actions are all defined differently on all major sections of the product. We need to standardize these implementations with a single direction.

Rules:

1. Generally, a 'Create' primary action should exist on an entity's index page
2. In rare cases, a primary action may not exist, in which case any action should be a secondary button (for example, on the extensions page)
3. If a primary action exists, it should be a primary button
4. The label for this action should be verb + entity name

• Batch changes should add the entity name so that the button reads "Create batch change"
  • https://github.com/sourcegraph/sourcegraph/issues/27772
• Code insights should change the button name to "Create code insight" and change the button type to primary
  • https://github.com/sourcegraph/sourcegraph/pull/27616
• Code monitors should add a primary button with the name "Create code monitor" when no monitors exist
  • It should also change the empty state button to be a secondary button
  • When a code monitor exists, the button should read "Create code monitor"
  • https://github.com/sourcegraph/sourcegraph/issues/27776

Design resources: https://www.figma.com/file/ZzWVJSMGKYDMLbfIAqWCvX/Consistency-of-primary-actions?node-id=0%3A1