Functional improvements + bug squashing

[teresaqhoang]

Motivation and Context

A variety of functional improvements, including:

• Adding supplement to ExternalInformation section of plan to help provide model more guidance.
• Adding tabs in prompt dialog to support view of raw rendered prompt content
• Fixing a bug to support .md file import
• In formatted prompt, added explicit message to indicate no relevant document memories were used in rendered prompt
• Note "Coming soon" for planner token usage
• Frontload custom plugins when they're added
• Align Login View
• Header corner items won't show on app launch up pages (i.e., backend probe page)

Description + Screenshots

New login view

Update prompt dialog with tabs

Support .md files

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines and the pre-submission formatting script raises no violations
  - [ ] All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄