Add Field for the Directory Intro Paragraph

[glenn-sorrentino]

This PR adds a new feature that allows administrators to set a customizable introductory description for the user directory. This introductory text appears at the top of the directory page, providing context or guidelines to visitors and enhancing user engagement.

Key Changes:

• Introduced a DIRECTORY_INTRO setting in OrganizationSetting to store the introductory text for the user directory.
• Added a form in the admin settings page that allows administrators to update the DIRECTORY_INTRO text.
• Implemented a new route update_directory_intro_text in the settings module to handle updates to the introductory text by admins.
• Modified the directory template to display the DIRECTORY_INTRO text at the top of the directory page for all users.
• Added unit tests to verify that the introductory text can be updated successfully and is displayed correctly on the directory page.

[brassy-endomorph]

A few nits but mostly a note about preventing XSS. Data needs to be escaped at display time always unless we can guaranteed it is safe. We usually can't. A rule of thumb is "user input is lava" and we want to handle it very very carefully. We don't even mark ints or UUIDs as safe when we display them even though we can assume quite strongly that they're always going to be safe. HTML that users input is never going be that, so when we pull from the DB it always needs to be cleaned/sanitized and then marked with a safe type.

[brassy-endomorph]

Sorry, still some changes, one of them being my fault for pointing you a little in the wrong direction :/