Extend Entity/Message with Header

[ebollens]

A use case raised by IT Services includes the need for a header bar on the message entity.

An example as posed by @melcassio:

Of course, we'll have rounded corners by default, but besides that, the rest clearly fits the current message entity, with the exception that we don't actually have a header structure yet for messages.

The markup to power this might be something like:

<aside class="message [connotation]">
  <header>
    <h1>Message Title Bar</h1>
  </header>
  <p>Some text related to the error</p>
  <p>Some more text</p>
</aside>

Similarly, we should support multiple lines such as:

<aside class="message [connotation]">
  <header>
    <h1>Message Primary Title</h1>
    <h2>Message Secondary Title</h2>
  </header>
  <p>Some text related to the error</p>
  <p>Some more text</p>
</aside>

[ebollens]

A working implementation in an extension:

• https://github.com/ucla/WebBlocks-kit-core/blob/master/extension/blocks/message/_header-variables.scss
• https://github.com/ucla/WebBlocks-kit-core/blob/master/extension/blocks/message/_header.scss