Allow configuring of banner for booking demo sessions

[midigofrank]

Description

This PR makes it possible to book for demo. When the user submits the form, they get a permanent redirect to the configured calendly url

Validation steps

This should be validated from the billing app. For standalone testing, you can enable the banner by changing the config

The banner is persistent across most pages (if not all) with the exception of the workflow edit page.

Notes for the reviewer

LayoutComponents.page_content/1 previously defined a slot named :banner. All pages that needed to show the banner was doing this:

<LayoutComponents.page_content>
  <:banner>
    <%= if assigns[:banner] do %>
      <%= Phoenix.LiveView.TagEngine.component(
        @banner.function,
        @banner.attrs,
        {__ENV__.module, __ENV__.function, __ENV__.file, __ENV__.line}
      ) %>
    <% end %>
  </:banner>
</LayoutComponents.page_content>

This was repetitive, so most of the changes you'll find are those that do this instead:

<LayoutComponents.page_content banner={assigns[:banner]}>
</LayoutComponents.page_content>

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

You can read more details in our Responsible AI Policy

Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

Attention: Patch coverage is 96.92308% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.73%. Comparing base (fbf90b7) to head (8329ef4).

Files with missing lines	Patch %	Lines
lib/lightning/config.ex	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3035      +/-   ##
==========================================
+ Coverage   91.71%   91.73%   +0.02%     
==========================================
  Files         348      349       +1     
  Lines       12956    13013      +57     
==========================================
+ Hits        11882    11937      +55     
- Misses       1074     1076       +2     

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