Add new data extensions header UI elements

[robertbrignull]

Updates the rest of the data extensions editor header to match the new designs.

• Application mode
  
• Framework mode
  

Now there are a few deviations from the original designs here:

• We're not going to do the "Hide open source dependencies" checkbox for now.
• We can't show "dependency_name@version" in framework mode because we don't know that information.
• We need to keep the "switch mode" button, although its text has been updated.
• The "open database" button shows the database source archive in the VS Code file explorer.
• I've kept the "open extension pack" button so there's still a way to locate those files.
  • Note that we may change its text in the future to remove the reference of "extension pack".
  • We could consider putting this on each individual section instead, so there's a way of linking to each model file directly.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

LGTM