Better campaign creation experience

This should be broken down into more specific issues with an MR for each of those specific issues. Each one is a rough idea and not necessarily a good idea. The point is that there is a lot of room to improve the process.

• (markdown + wysiwyg) or (markdown + preview) editor instead of forcing people to learn markdown
• Improve the quality of markdown parsing / presentation. For example currently there often end up being strange line breaks and other unexpected formatting errors. The allowed markdown is also quite limited (e.g. no tables).
• Include a spellchecker in whatever process there is
• Allow preview and corrections in whatever process is used.

Note: Any effort to make active campaigns editable will be rejected. The only exception would be something that tracks and makes edits very visible / easy to understand.