Add more rich text formatting options in the job submission editor

### Is your feature request related to a problem? Please describe
Currently, the job submission form has a very limited WYSIWG editor for the job description.

### Describe the solution you'd like
Extend the capability of this editor by adding more formatting options like headings.

### Describe alternatives you've considered
Support HTML code formatting in the editor.

### Additional context
3523513-zen

Start by locating the job submission form and its WYSIWYG editor in the WP Job Manager codebase, then inspect how the current formatting controls are configured. Compare the requested headings and HTML code formatting with the editor's existing capabilities and verify that the new options work in job descriptions.

Written by the indexing model from the issue text.